Name: Heart Rate Graph
Author: flyingflo

This data field shows the current heart rate and draws a graph with recent values in the background.

- draws history, one pixel per second.
- configurable min, max, and threshold.
- three graph colors: below threshold, above threshold, above max.
- dedicated color palette for day and night mode.
- optimized battery consumption: keeps the graph in an offscreen bitmap to avoid full redraws (full redraws are 10x slower).

Testing settings:

- Demo mode: shows a linear ramp to test color palettes.
- Timing: Display the graph redraw time in milliseconds in the field label. It's about 20ms on the Edge 820.

Color customization:

The graph colors can be customized in the settings form. 
For each palette, bright and dark, three colors are used. 
Specify them as a hexadecimal RGB numeric value. Each 
color channel is repressented by one byte. E.g. 0xff0000 is 
bright red, 0x0000ff is bright blue.
The Garmin developer documentation has a table of available colors.
https://developer.garmin.com/connect-iq/user-experience-guide/page-layout/#bitmaps
Invalid values cause a silent fallback to the default color, because there is no good
way to signal an error for a CIQ datafield.