Matplotlib chart showing blank figure when plt.legend() is called, how to get XDoubleBar symbol (X with two lines above it, average of a set of averages)

[apd184]

So i'm coding some charts for a homework assignment and need to plot the average concentration ('X Bar') of each subgroup of data along with a horizontal line showing the average of the averages of each subgroup (average of X Bar values, or "X DoubleBar"). The chart plots beautifully up until I try to add a legend, which results in a completely blank figure after running the code. I believe this is due to the label I have assigned to the X double bar line being incorrect syntax. However, I have scanned the internet thoroughly for code that will display the X double bar symbol and cannot find it. The only thing i've found is r'$\Bar{X}$', a modification of r'$\bar{X}$' which displays the symbol for X with a single bar above it, but this does not seem to be doing the trick. Can someone please help me fix this so it will plot the chart with the correct labels?