[Platform]: Phewas and Manhattan plot fixes

[gjmcn]

Description

Fixes for Manhattan plot (study page) and PheWas plot (variant page):

• Both plots

  • switch to computing y values directly and using linear y-scale in plots - very small p-values (less thanNumber.MIN_NUMBER) and use of log scales was causing various issues
  • use dominant-baseline instead of alignment-baseline with labels for consistent behavior across browsers

• Phewas plot:

  • update margins
  • edit therapeutic area labels so shorter (after discussion with @d0choa)
  • fix bug with uncategorised values in PheWas plot
  • add legend

Issue: #3537, #3502

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Checked no study and variant pages.

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have made corresponding changes to the documentation