If you don’t want to spend on a Pixel for GrapheneOS, have you considered CalyxOS or LineageOS? CalyxOS only supports a few devices outside of the Pixel range, while LineageOS covers a broader range.
I can’t vouch for them personally, as I haven’t used them, but I’ve heard good things about them.
What about a 3D chart, with the 4th axis being portrayed via the Hue value of the point on the chart? That would make it somewhat readable.