Fix exception when using :group-by in histogram

[glts]

huri.plot/histogram supports grouping with :group-by. When using grouping and :show-mean? is enabled at the same time, an exception is thrown and no plot is generated.

Since :show-mean? is true by default, this error is very easily triggered.

The change proposed here removes a (mistaken, as far as I can tell) reference to group-by in the drawing of the mean line.