Adding text using matplotlib - matplotlib

Question

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)

Answer 1

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)