`render_combined` implicitly relies on axis limits being the same for x and y axis.

[Schaechtle]

For an exercise in section 5, we request the user to do construct their own dataset and then render it with render_combined, which relies on render_trace and render_sine_trace. Both functions run:

ax[:set_ylim]((xmin, xmax))

If a user constructs a dataset where x and y axis limits don't agree, it won't be plotted fully (or at all).