Grapher: Increase opacity to 100% where appropriate

[sophiamersmann]

• Some of our chart types use 80% opacity to make the grid lines in the background visible
• Marwa suggested using 100% opacity instead and drawing grid lines at a darker colour with some opacity on top
• We could do this for all chart types where visual elements don't overlap (e.g. doing this for scatters would be inappropriate)
• We would need to check whether the changes are actually nice and not, for example, visually too loud (Daniel mentioned that he looked at Stacked area charts and found it quite intense)
  • We could try choosing lighter colours for larger areas or reduce area colours slightly in saturation

[sophiamersmann]

Marwa's designs and bullet point lists of low-hanging fruit: https://www.figma.com/design/2tH8dIkXurkktvumTneZZq/Grapher-UI-1.1?node-id=529-1204&t=FDZiC27W8K1h4kvT-1