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I am trying to build a graph using matplotlib, and I am having trouble placing descriptive text on the graph itself.
My y values range from .9 to 1.65, and x ranges from the dates 2001 to 2021 and are sourced from a datetime series.
Here are the basics of what I am working with:
fig, ax = plt.subplots(figsize=(10,7))
I know that I have to use ax.text() to place any text, but whenever I try to enter basically any values for the x and y coordinates of the text, the entire graph disappears when I re-run the cell. I have plotted the following line, but if I use the same coordinates in ax.text(), I get the output I just described. Why might this be happening?
plt.axhline(y=1.19, xmin=.032, xmax=.96)
By default, the y argument in the axhline method is in data coordinates, while the xmin and xmax arguments are in axis coordinates, with 0 corresponding to the far left of the plot, and 1 corresponding to the far right of the plot. See the axhline documentation for more information.
On the other hand, both the x and y arguments used in the text method are in data coordinates, so you position text relative to the data. However, you can change this to axis coordinates using the transform parameter. By setting this to ax.transAxes, you actually indicate that the x and y arguments should be interpreted as axis coordinates, again with 0 being the far left (or bottom) of the plot, and 1 being the far right (or top) of the plot. In this case, you would use ax.text as follows:
ax.text(x, y, 'text', transform=ax.transAxes)
Again, see the text documentation for more information.
However, it sounds like you might want to combine data and axis coordinates to place your text, because you want to reuse the arguments from axhline for your text. In this case, you need to create a transform object that interprets the x coordinate as axis coordinate, and the y coordinate as data coordinate. This is also possible by creating a blended transformation. For example:
import matplotlib.transforms as transforms
# create your ax object here
trans = transforms.blended_transform_factory(x_transform=ax.transAxes, y_transform=ax.transData)
ax.text(x, y, 'text', transform=trans)
See the Blended transformations section of the transformations tutorial for more information.
In short, you can refer to the following figure to compare the results of these various transformations:
import matplotlib.pyplot as plt
import matplotlib.transforms as transforms
fig, ax = plt.subplots()
ax.set_xlim(0, 2)
ax.set_ylim(0, 2)
# note that the line is plotted at y=1.5, but between x=1.6 and x=1.8
# because xmin/xmax are in axis coordinates
ax.axhline(1.5, xmin=.8, xmax=.9)
# x and y are in data coordinates
ax.text(0.5, 0.5, 'A')
# here, x and y are in axis coordinates
ax.text(0.5, 0.5, 'B', transform=ax.transAxes)
trans = transforms.blended_transform_factory(x_transform=ax.transAxes, y_transform=ax.transData)
# here, x is in axis coordinates, but y is in data coordinates
ax.text(0.5, 0.5, 'C', transform=trans)
I want to add a scale indicator to a plot like the one labelled '10kpc' in the (otherwise) empty plot below. So basically, the axis use one unit of measure and I want to indicate a length in the plot in a different unit. It has to have the same style as below, i.e. a |----| bar with text above.
Is there a convenient way in matplotlib to do that or do I have to draw three lines (two small vertical, one horizontal) and add the text? An ideal solution would not even require me to set coordinates in the data dimensions, i.e. I just say something along the line of horizontalalignment='left', verticalalignment='bottom', transform=ax.transAxes and specify only the width in data coordinates.
I fought with annotate() and arrow() and their documentations for quiet a bit until I concluded, they were not exactly useful, but I might be wrong.
Edit:
The code below is the closest, I have come so far. I still don't like having to specify the x-coordinates in the data coordinate system. The only thing I want to specify in data is the width of the bar. The rest should be placed in the plot system and ideally the bar should be placed relative to the text (a few pixels above).
import matplotlib.pyplot as plt
import matplotlib.transforms as tfrms
plt.imshow(somedata)
plt.colorbar()
ax = plt.gca()
trans = tfrms.blended_transform_factory( ax.transData, ax.transAxes )
plt.errorbar( 5, 0.06, xerr=10*arcsecperkpc/2, color='k', capsize=5, transform=trans )
plt.text( 5, 0.05, '10kpc', horizontalalignment='center', verticalalignment='top', transform=trans )
Here is a code that adds a horizontal scale bar (or scale indicator or scalebar) to a plot. The bar's width is given in data units, while the height of the edges is in fraction of axes units.
The solution is based on an AnchoredOffsetbox, which contains a VPacker. The VPacker has a label in its lower row, and an AuxTransformBox in its upper row.
The key here is that the AnchoredOffsetbox is positioned relative to the axes, using the loc argument similar to the legend positioning (e.g. loc=4 denotes the lower right corner). However, the AuxTransformBox contains a set of elements, which are positioned inside the box using a transformation. As transformation we can choose a blended transform which transforms x coordinates according to the data transform of the axes and y coordinates according to the axes transform. A tranformation which does this is actually the xaxis_transform of the axes itself. Supplying this transform to the AuxTransformBox allows us to specify the artists within (which are Line2Ds in this case) in a useful way, e.g. the line of the bar will be Line2D([0,size],[0,0]).
All of this can be packed into a class, subclassing the AnchoredOffsetbox, such that it is easy to be used in an existing code.
import matplotlib.pyplot as plt
import matplotlib.offsetbox
from matplotlib.lines import Line2D
import numpy as np; np.random.seed(42)
x = np.linspace(-6,6, num=100)
y = np.linspace(-10,10, num=100)
X,Y = np.meshgrid(x,y)
Z = np.sin(X)/X+np.sin(Y)/Y
fig, ax = plt.subplots()
ax.contourf(X,Y,Z, alpha=.1)
ax.contour(X,Y,Z, alpha=.4)
class AnchoredHScaleBar(matplotlib.offsetbox.AnchoredOffsetbox):
""" size: length of bar in data units
extent : height of bar ends in axes units """
def __init__(self, size=1, extent = 0.03, label="", loc=2, ax=None,
pad=0.4, borderpad=0.5, ppad = 0, sep=2, prop=None,
frameon=True, linekw={}, **kwargs):
if not ax:
ax = plt.gca()
trans = ax.get_xaxis_transform()
size_bar = matplotlib.offsetbox.AuxTransformBox(trans)
line = Line2D([0,size],[0,0], **linekw)
vline1 = Line2D([0,0],[-extent/2.,extent/2.], **linekw)
vline2 = Line2D([size,size],[-extent/2.,extent/2.], **linekw)
size_bar.add_artist(line)
size_bar.add_artist(vline1)
size_bar.add_artist(vline2)
txt = matplotlib.offsetbox.TextArea(label, minimumdescent=False)
self.vpac = matplotlib.offsetbox.VPacker(children=[size_bar,txt],
align="center", pad=ppad, sep=sep)
matplotlib.offsetbox.AnchoredOffsetbox.__init__(self, loc, pad=pad,
borderpad=borderpad, child=self.vpac, prop=prop, frameon=frameon,
**kwargs)
ob = AnchoredHScaleBar(size=3, label="3 units", loc=4, frameon=True,
pad=0.6,sep=4, linekw=dict(color="crimson"),)
ax.add_artist(ob)
plt.show()
In order to achieve a result as desired in the question, you can set the frame off and adjust the linewidth. Of course the transformation from the units you want to show (kpc) into data units (km?) needs to be done by yourself.
ikpc = lambda x: x*3.085e16 #x in kpc, return in km
ob = AnchoredHScaleBar(size=ikpc(10), label="10kpc", loc=4, frameon=False,
pad=0.6,sep=4, linekw=dict(color="k", linewidth=0.8))
I want to change the scale of x axis in matplotlib in python. I am using following code.
df.iloc[:,5:9].plot(kind="density",subplots=True,layout=(2,2),sharex=False)
I want to have different X axis scale for all X axis. What I tried is following
fig=plt.figure(figsize=(15,15))
ax1=fig.add_subplot(2,2,1)
ax2=fig.add_subplot(2,2,2)
ax2=fig.add_subplot(2,2,3)
ax2=fig.add_subplot(2,2,4)
ax1.set_xticks(np.arrange(1,5000,500))
ax2.set_xticks(np.arrange(1,5000,500))
ax3.set_xticks(np.arrange(1,5000,500))
ax4.set_xticks(np.arrange(1,5000,500))
But, when I run this I get empty figure.
You need to operate on the axes in which the plots live, not in some newly created empty axes.
axes = df.plot(kind="density",subplots=True,layout=(2,2),sharex=False)
axes[0,0].set_xticks(...)
I've drawn a plot that looks something like the following:
It was created using the following code:
import numpy as np
import pandas as pd
import matplotlib as mpl
import matplotlib.pyplot as plt
# 1. Plot a figure consisting of 3 separate axes
# ==============================================
plotNames = ['Plot1','Plot2','Plot3']
figure, axisList = plt.subplots(len(plotNames), sharex=True, sharey=True)
tempDF = pd.DataFrame()
tempDF['date'] = pd.date_range('2015-01-01','2015-12-31',freq='D')
tempDF['value'] = np.random.randn(tempDF['date'].size)
tempDF['value2'] = np.random.randn(tempDF['date'].size)
for i in range(len(plotNames)):
axisList[i].plot_date(tempDF['date'],tempDF['value'],'b-',xdate=True)
# 2. Create a new single axis in the figure. This new axis sits over
# the top of the axes drawn previously. Make all the components of
# the new single axis invisibe except for the x and y labels.
big_ax = figure.add_subplot(111)
big_ax.set_axis_bgcolor('none')
big_ax.set_xlabel('Date',fontweight='bold')
big_ax.set_ylabel('Random normal',fontweight='bold')
big_ax.tick_params(labelcolor='none', top='off', bottom='off', left='off', right='off')
big_ax.spines['right'].set_visible(False)
big_ax.spines['top'].set_visible(False)
big_ax.spines['left'].set_visible(False)
big_ax.spines['bottom'].set_visible(False)
# 3. Plot a separate figure
# =========================
figure2,ax2 = plt.subplots()
ax2.plot_date(tempDF['date'],tempDF['value2'],'-',xdate=True,color='green')
ax2.set_xlabel('Date',fontweight='bold')
ax2.set_ylabel('Random normal',fontweight='bold')
# Save plot
# =========
plt.savefig('tempPlot.png',dpi=300)
Basically, the rationale for plotting the whole picture is as follows:
Create the first figure and plot 3 separate axes using a loop
Plot a single axis in the same figure to sit on top of the graphs
drawn previously. Label the x and y axes. Make all other aspects of
this axis invisible.
Create a second figure and plot data on a single axis.
The plot displays just as I want when using jupyter-notebook but when the plot is saved, the file contains only the second figure.
I was under the impression that plots could have multiple figures and that figures could have multiple axes. However, I suspect I have a fundamental misunderstanding of the differences between plots, subplots, figures and axes. Can someone please explain what I'm doing wrong and explain how to get the whole image to save to a single file.
Matplotlib does not have "plots". In that sense,
plots are figures
subplots are axes
During runtime of a script you can have as many figures as you wish. Calling plt.save() will save the currently active figure, i.e. the figure you would get by calling plt.gcf().
You can save any other figure either by providing a figure number num:
plt.figure(num)
plt.savefig("output.png")
or by having a refence to the figure object fig1
fig1.savefig("output.png")
In order to save several figures into one file, one could go the way detailed here: Python saving multiple figures into one PDF file.
Another option would be not to create several figures, but a single one, using subplots,
fig = plt.figure()
ax = plt.add_subplot(611)
ax2 = plt.add_subplot(612)
ax3 = plt.add_subplot(613)
ax4 = plt.add_subplot(212)
and then plot the respective graphs to those axes using
ax.plot(x,y)
or in the case of a pandas dataframe df
df.plot(x="column1", y="column2", ax=ax)
This second option can of course be generalized to arbitrary axes positions using subplots on grids. This is detailed in the matplotlib user's guide Customizing Location of Subplot Using GridSpec
Furthermore, it is possible to position an axes (a subplot so to speak) at any position in the figure using fig.add_axes([left, bottom, width, height]) (where left, bottom, width, height are in figure coordinates, ranging from 0 to 1).
I'm having trouble giving colorbars to a grid of line plots in Matplotlib.
I have a grid of plots, which each shows 64 lines. The lines depict the penalty value vs time when optimizing the same system under 64 different values of a certain hyperparameter h.
Since there are so many lines, instead of using a standard legend, I'd like to use a colorbar, and color the lines by the value of h. In other words, I'd like something that looks like this:
The above was done by adding a new axis to hold the colorbar, by calling figure.add_axes([0.95, 0.2, 0.02, 0.6]), passing in the axis position explicitly as parameters to that method. The colorbar was then created as in the example code here, by instantiating a ColorbarBase(). That's fine for single plots, but I'd like to make a grid of plots like the one above.
To do this, I tried doubling the number of subplots, and using every other subplot axis for the colorbar. Unfortunately, this led to the colorbars having the same size/shape as the plots:
Is there a way to shrink just the colorbar subplots in a grid of subplots like the 1x2 grid above?
Ideally, it'd be great if the colorbar just shared the same axis as the line plot it describes. I saw that the colorbar.colorbar() function has an ax parameter:
ax
parent axes object from which space for a new colorbar axes will be stolen.
That sounds great, except that colorbar.colorbar() requires you to pass in a imshow image, or a ContourSet, but my plot is neither an image nor a contour plot. Can I achieve the same (axis-sharing) effect using ColorbarBase?
It turns out you can have different-shaped subplots, so long as all the plots in a given row have the same height, and all the plots in a given column have the same width.
You can do this using gridspec.GridSpec, as described in this answer.
So I set the columns with line plots to be 20x wider than the columns with color bars. The code looks like:
grid_spec = gridspec.GridSpec(num_rows,
num_columns * 2,
width_ratios=[20, 1] * num_columns)
colormap_type = cm.cool
for (x_vec_list,
y_vec_list,
color_hyperparam_vec,
plot_index) in izip(x_vec_lists,
y_vec_lists,
color_hyperparam_vecs,
range(len(x_vecs))):
line_axis = plt.subplot(grid_spec[grid_index * 2])
colorbar_axis = plt.subplot(grid_spec[grid_index * 2 + 1])
colormap_normalizer = mpl.colors.Normalize(vmin=color_hyperparam_vec.min(),
vmax=color_hyperparam_vec.max())
scalar_to_color_map = mpl.cm.ScalarMappable(norm=colormap_normalizer,
cmap=colormap_type)
colorbar.ColorbarBase(colorbar_axis,
cmap=colormap_type,
norm=colormap_normalizer)
for (line_index,
x_vec,
y_vec) in zip(range(len(x_vec_list)),
x_vec_list,
y_vec_list):
hyperparam = color_hyperparam_vec[line_index]
line_color = scalar_to_color_map.to_rgba(hyperparam)
line_axis.plot(x_vec, y_vec, color=line_color, alpha=0.5)
For num_rows=1 and num_columns=1, this looks like: