plot_heatmaps / plot_heatmaps_logarithmic crash on their own default dimensions=None

Both functions declare dimensions=None as the default (src/graphing/plotting.py:633
and :759), but get_heatmap_data immediately does len(dimensions)
(src/graphing/heatmap.py:207):
TypeError: object of type 'NoneType' has no len()
plot_heatmaps_logarithmic fails slightly differently: TypeError: '>' not supported
between instances of 'float' and 'NoneType'.

get_heatmap_data already has a friendly error path printing the expected dimensions
format when the list is the wrong length — None just never reaches it. Either guard
for None there, or make dimensions a required positional argument since there's no
usable default.

Start with plot_heatmaps and plot_heatmaps_logarithmic in src/graphing/plotting.py at lines 633 and 759, then trace their dimensions handling into get_heatmap_data in src/graphing/heatmap.py:207. Check both calls with dimensions=None and compare them with the existing wrong-length error path. Done means the default behavior no longer raises the reported TypeError and dimensions handling is consistent with the chosen API behavior.

Written by the indexing model from the issue text.