Issue checking x-limits of lines without additional plotted data.

[marty-larocque]

Some methods of plotting lines with Matplotlib do not yield xy_data. Specifically, base.get_xy() returns None when lines are plotted with some methods.

This causes an issue when calling base.assert_line(), because assert_line() checks that a line spans beyond the x-limits of the plotted data. If no data is plotted (i.e. nothing is plotted other than the line), then these x-limits cannot be determined and an error is thrown.

Here is a gist demonstrating this issue:
https://gist.github.com/ryla5068/53762a1b5770ffcb945793274d842904

This is an uncommon case, and could probably go unaddressed. We should decide whether we want to address this issue or not.

[lwasser]

@ryla5068 can you provide some clarification of what methods used to plot return a None value from MPC? the gist doesn't have much information in it - it it just a single plot with a single 1:1 line. What are the conditions that yield a none value? I would assume MPC is looking in the wrong place for the line if it returns none but a line is clearly drawn.

[lwasser]

assert_lines checks for 3 things

1. the slope
2. the intercept
3. the xlims are beyond those of the data in the plot

[marty-larocque]

This happens when two conditions are met:

1. A line is plotted
2. No points are plotted (e.g. by using plt.scatter())
3. assert_line() checks that the xlims are beyond those of the data in the plot

PR #239 does not resolve this issue completely, but it changes assert_line() so that it does not check xlims by default. This makes this issue a far less common edge case.