Matplotlib - axvspan vs subplots

Question

I'm writing a pythonic script for a coastal engineering application which should output, amongst other things, a figure with two subplots.

The problem is that I would like to shade a section of both subplots using plt.axvspan() but for some reason it only shades one of them.

Please find below an excerpt of the section of the code where I set up the plots as well as the figure that it's currently outputting (link after code).

Thanks for your help, and sorry if this is a rookie question (but it just happens that I am indeed a rookie in Python... and programming in general) but I couldn't find an answer for this anywhere else.

Feel free to add any comments to the code.

# PLOTTING
# now we generate a figure with the bathymetry vs required m50 and another figure with bathy vs Hs

#1. Generate plots

fig = plt.figure() # Generate Figure
ax = fig.add_subplot(211) # add the first plot to the figure.
depth = ax.plot(results[:,0],results[:,1]*-1,label="Depth [mDMD]") #plot the first set of data onto the first set of axis.
ax2 = ax.twinx() # generate a secondary vertical axis with the same horizontal axis as the first
m50 = ax2.plot(results[:,0],results[:,6],"r",label="M50 [kg]") # plot the second set of data onto the second vertical axis
ax3 = fig.add_subplot(212) # generate the second subplot
hs = ax3.plot(results[:,0],results[:,2],"g",label="Hs(m)")

#Now we want to find where breaking starts to occur so we shade it on the plot.
xBreakingDistance = results[numpy.argmax(breakingIndex),0]

# and now we plot a box from the origin to the depth of breaking.

plt.axvspan(0,xBreakingDistance,facecolor="b",alpha=0.1) # this box is called a span in matplotlib (also works for axhspan)

# and then we write BREAKING ZONE in the box we just created

yLimits = ax.get_ylim() # first we get the range of y being plotted
yMiddle = (float(yLimits[1])-float(yLimits[0])) / 2 + yLimits[0] # then we calculate the middle value in y (to center the text)
xMiddle = xBreakingDistance / 2 # and then the middle value in x (to center the text)
#now we write BREAKING ZONE in the center of the box.
ax.text(xMiddle,yMiddle,"BREAKING ZONE",fontweight="bold",rotation=90,verticalalignment="center",horizontalalignment="center")

#FIGURE FORMATTING
ax.set_xlabel("Distance [m]") # define x label
ax.set_ylabel("Depth [mDMD]") # define y label on the first vertical axis (ax)
ax2.set_ylabel("M50 [kg]") # define y label on the second vertical axis (ax2)
ax.grid() # show grid
ax3.set_xlabel("Distance[m]") #define x label
ax3.set_ylabel("Hs[m]") # define y label
ax3.grid()
plt.tight_layout() # minimize subplot labels overlapping

# generating a label on a plot with 2 vertical axis is not very intuitive. Normally we would just write ax.label(loc=0)
combined_plots = depth+m50 #first we need to combine the plots in a vector
combined_labels = [i.get_label() for i in combined_plots] # and then we combine the labels
ax.legend(combined_plots,combined_labels,loc=0) # and finally we plot the combined_labels of the combined_plots
plt.savefig("Required M50(kg) along the trench.png",dpi=1000)
plt.close(fig)

Output Figure:

Answer 1

By just calling plt.axvspan , you are telling matplotlib to create the axvspan on the currently active axes (ie in this case, the last one you created, ax3 )

You need to plot the axvspan on both of the axes you would like for it to appear on. In this case, ax and ax3 .

So, you could do:

ax.axvspan(0,xBreakingDistance,facecolor="b",alpha=0.1)    
ax3.axvspan(0,xBreakingDistance,facecolor="b",alpha=0.1)

or in one line:

[this_ax.axvspan(0,xBreakingDistance,facecolor="b",alpha=0.1) for this_ax in [ax,ax3]]

Answer 2

It's difficult to analyze your code and not being able to reproduce it. I advise you to build a minimal example. In any case notice that you are calling "plt.axvspan(" which is general call to the library.

You need to specifically state that you want this in both "ax" and "ax2" (i think).

Also if you need more control consider using Patches (I don't know axvspan):

import matplotlib.pyplot as plt
import matplotlib.patches as patches

fig1 = plt.figure()
ax1 = fig1.add_subplot(111, aspect='equal')
ax1.add_patch(
    patches.Rectangle(
        (0.1, 0.1),   # (x,y)
        0.5,          # width
        0.5,          # height
    )
)
fig1.savefig('rect1.png', dpi=90, bbox_inches='tight')

See that call to "ax1" in the example? Just make something similar to yours. Or just add axvspan to each of your plots.