Decouple values and graphs?

[usbalbin]

As is now, we have metrics which both specifies a value to send to the host and the graph to plot. Let's consider we have one metric for voltage and one metric for current. What if we want a third graph for power that is just the multiplication of voltage * current?

If we split the values and the plots we can let there be more graphs than there are values etc.

For example (assuming #8)

let duty = make_setting!(DUTY: u16 = 0, 0..=90, 1);
let v_target = make_setting!(TARGET: u16 = 0, 42..=123, 1);

let voltage = make_metric!(V: u16 = 0);
let current = make_metric!(I: u16 = 0);

make_graph!(Voltage = "V / 4095 * 42"); // Creates a graph with title `Voltage`
make_graph!(Current = "I / 4095 * 123"); // One metric
make_graph!(Power = "(V * I) / 1337"); // Two metrics
make_graph!(Duty = "DUTY"); // One setting
make_graph!(Error = "TARGET - V"); // Expression with both one setting and one metric