[scatterplot] ignore inappropriate settings when using string-based attributes

[JobLeonard]

By definition, string-based attributes, like comments or cell types, are categorical in nature. So let's default to sensible options and interpretations when using them:

• X/Y attributes:
  • disable the log button
  • ignore whatever log setting is selected
• Color Attribute:
  • don't show heat map/catgorical selection buttons
  • always use categorical colouring