Legend transparent to horizontal grid in matplotlib

Question

I'm working with the following class:

import numpy as np
import matplotlib
matplotlib.use('Qt4Agg')
import matplotlib.pyplot as plt
import matplotlib.ticker as plticker

class matplotLIV():

    def __init__(self, BaseFilename, temperatures, length=None, width=None, area=None, title = '', ylim=None):
        self.BaseFilename = BaseFilename
        self.temperatures = temperatures
        if length and width:
            self.length = length
            self.width = width
            self.area = length*width*1e-5
        else:
            self.area = area
        self.title = title
        self.ylim = ylim

        filenames = [("%s_%sK.txt" % (self.BaseFilename, str(temp)), temp) for temp in self.temperatures]
        self.rawData = [(np.loadtxt(fname), temp) for fname, temp in filenames]
        self.colors = colors = ['#1b9e77', '#d95f02', '#7570b3', '#e7298a', '#e6ab02', '#a6761d', '#666666']

        self.maxValueRow = (0,0,0)

    def plot(self):

        self.fig = plt.figure()
        self.ax1 = self.fig.add_subplot(111)
        ax1 = self.ax1
        ax1.tick_params(bottom='off')
        ax1.xaxis.tick_top()
        self.ax2 = ax1.twinx()
        ax2 = self.ax2
        self.ax3 = ax2.twiny()
        ax3 = self.ax3
        ax3.xaxis.tick_bottom()

        ax1.set_xlabel("current / A")
        ax1.xaxis.set_label_position('top')
        ax1.set_ylabel("voltage / V")
        ax2.set_ylabel("light intensity / arb. u.")
        ax3.set_xlabel(r'current density / $\mathregular{Acm^{-2}}$')
        ax3.xaxis.set_label_position('bottom')

        for i, (datafile, label) in enumerate(self.rawData):
            self.checkMaxValues(datafile)
            ax1.plot( datafile[:,0], datafile[:,1], color=self.colors[i], label='%sK' % str(label))
            ax2.plot( datafile[:,0], datafile[:,2], color=self.colors[i], label='%sK' % str(label), linewidth=2)


        ax1.margins(x=0)

        ax1.grid(True, axis='y')
        ax3.grid(True)

        start, end = ax1.get_xlim()

        self.setAxesScale(ax1, ax2)
        if self.ylim:
            ax2.set_ylim(top=self.ylim)

        ax3.set_xlim(start/self.area, end/self.area)
        leg = ax2.legend(loc='upper left')

        self.fig.suptitle(self.title, y=0.98, weight='bold')
        self.fig.subplots_adjust(top=0.86)

        loc = plticker.MultipleLocator(base=20.0) # this locator puts ticks at regular intervals
        ax3.xaxis.set_major_locator(loc)

    def checkMaxValues(self, data):
        maxInd = data.argmax(axis=0)[2]
        if data[maxInd][2] > self.maxValueRow[2]:
            self.maxValueRow = data[maxInd]

    def setAxesScale(self, ax1, ax2):
        yrange = ax1.get_ylim()
        y1Fraction = self.maxValueRow[1]/yrange[1]
        y2Fraction = y1Fraction - 0.02
        ax2.set_ylim(top=self.maxValueRow[2]/y2Fraction)

    def show(self):
        plt.savefig(self.BaseFilename + '.pdf')        
        plt.show()

which you can run with this sample code:

import matplotLIV as mpliv

######## configuration

BaseFilename = "testdata"
temperatures = (5,)

area = 1e-8

######## end of configuration

liv = mpliv.matplotLIV(BaseFilename, temperatures, area=area)
liv.plot()
liv.show()

on this file: http://pastebin.com/GMAC3mUu

The problem that I'm experiencing is that the legend is transparent to the grid. Oddly enough, it is only the vertical grid that you can see through the legend box:

Is this a bug? If not, how do I set the legend so it is NOT transparent?

Answer 1

The problem is the vertical grid is on ax3, and the legend is on ax2, so the grid is plotted after the legend.

One way around this is pasted below (just the section you need to modify). You need to plot the legend on ax3, and explicitly tell it which lines and labels you want.

    # make a list for the lines that you are plotting
    l1 = []
    l2 = []
    for i, (datafile, label) in enumerate(self.rawData):
        self.checkMaxValues(datafile)
        # Give your lines some names (l1,l2)
        l1+=ax1.plot( datafile[:,0], datafile[:,1], color=self.colors[i], label='%sK' % str(label))
        l2+=ax2.plot( datafile[:,0], datafile[:,2], color=self.colors[i], label='%sK' % str(label), linewidth=2)

    # Define which lines to put in the legend. If you want l1 too, then use lns = l1+l2
    lns = l2
    labs = [l.get_label() for l in lns]

    ax1.margins(x=0)

    ax1.grid(True, axis='y')
    ax3.grid(True)

    start, end = ax1.get_xlim()

    self.setAxesScale(ax1, ax2)
    if self.ylim:
        ax2.set_ylim(top=self.ylim)

    ax3.set_xlim(start/self.area, end/self.area)
    # Set the legend on ax3, not ax2
    leg = ax3.legend(lns,labs,loc='upper left')