matplotlib scatter plot using axes object in loop - matplotlib

Question

I am having trouble using Matplotlib to plot multiple series in a loop (Matplotlib 1.0.0, Python 2.6.5, ArcGIS 10.0). Forum research pointed me to application of an Axes object, in order to plot multiple series on the same plot. I see how this works well for data generated outside of a loop (sample scripts), but when I insert the same syntax and add the second series into my loop that pulls data from database, I get the following error:
": unsupported operand type(s) for -: 'NoneType' and 'NoneType' Failed to execute (ChartAge8)."
Below is my code - any suggestions or comments are much appreciated!
import arcpy
import os
import matplotlib
import matplotlib.pyplot as plt
#Variables
FC = arcpy.GetParameterAsText(0) #feature class
P1_fld = arcpy.GetParameterAsText(1) #score field to chart
P2_fld = arcpy.GetParameterAsText(2) #score field to chart
plt.subplots_adjust(hspace=0.4)
nsubp = int(arcpy.GetCount_management(FC).getOutput(0)) #pulls n subplots from FC
last_val = object()
#Sub-plot loop
cur = arcpy.SearchCursor(FC, "", "", P1_fld)
i = 0
x1 = 1 # category 1 locator along x-axis
x2 = 2 # category 2 locator along x-axis
fig = plt.figure()
for row in cur:
y1 = row.getValue(P1_fld)
y2 = row.getValue(P2_fld)
i += 1
ax1 = fig.add_subplot(nsubp, 1, i)
ax1.scatter(x1, y1, s=10, c='b', marker="s")
ax1.scatter(x2, y2, s=10, c='r', marker="o")
del row, cur
#Save plot to pdf, open
figPDf = r"path.pdf"
plt.savefig(figPDf)
os.startfile("path.pdf")

Answer 1

If what you want to do is plot several stuff reusing the same plot what you should do it create the figure object outside the loop and then plot to that same object everytime, something like this:
fig = plt.figure()
for row in cur:
y1 = row.getValue(P1_fld)
y2 = row.getValue(P2_fld)
i += 1
ax1 = fig.add_subplot(nsubp, 1, i)
ax1.scatter(x1, y1, s=10, c='b', marker="s")
ax1.scatter(x2, y2, s=10, c='r', marker="o")
del row, cur