add image- enhanced versions of histograms, bar- and pie-charts

[schanzer]

This largely follows the behavior of image-scatter-plot, so there shouldn't be (m)any surprises. A few things that I'm not sure about, however:

• I'm making the assumption that an image display for histograms, bar and pie charts will only ever be constructed from a single series. This is the same assumption made in image-scatter-plot as well, but I don't know enough to be confident that it's safe to make it here
• When rendering the live charts, everything looks great. But the resulting image doesn't include the new images! Is this because I'm running locally? If so, I would have seen the same thing with image-scatter-plot...but I don't remember if I did. As a result the included test fails -- @jpolitz can likely remind me what needed to happen in order to get the custom images into the resulting output.

Thanks in advance for taking a look!