HardwareReport: add clock drift plot

[IamPete1]

This adds a clock drift plot:

Based on:

https://github.com/ArduPilot/pymavlink/pull/938

This is much more noisy that I would have expected. I'm not sure why.

[IamPete1]

Results from some other logs:

[IamPete1]

This is a bad example:

It now sets the default range to +-1000PPM to such that normal jitter doesn't look so bad.