Fix setting contrast limits when the histogram widget is open

[p-j-smith]

Fixes #272

• pass a list to Line2D.set_xdata rather than a float (Line2D.set_xdata expects an iterable)
• add test for setting contrast limits when a histogram is plotted

[dstansby]

Thanks a lot for this long overdue fix! It has been a while since any development happened here, but all the tests seem to be running fine... before merging, could you add a new 2.1.1 section to the changelog (docs/changelog.rst) with a short entry for this bugfix?

[p-j-smith]

Thanks for the speedy review!

I've updated the changelog although I put it under 3.0.1 as the latest release is for 3.0.0.

Also, I've renamed 2.1.0 in the changelog as it looks like 2.1.0 was never released and it went straight to 3.0.0? Happy to change it back though if this isn't right

[dstansby]

Ah, it looks like I forgot to update the changelog in the docs for 3.0.0 🤦 - I'll do that in a separate PR, and then do a release