Add graph for history of JSON values

[tjorim]

Similar to #31, but let users plot values in json topics as well; instead of only plotting numeric topics.

[thomasnordquist]

How would you go about it?

I thought about maybe shallow scan (first hierarchical level) of the json and plot those numbers.
One might also need to be able to select the plotted fields.

Axis scaling might be difficult:
A Unix timestamp is rather big compared to a temperature reading, plotting both and auto-scale the axis would make the temperature reading insignificant.

Few thoughts/ideas:

• Add a "Plot" window that plots all values in their own graphs
• Add a combined plot but try to group/filter plots with similar magnitude
• Add a configuration for combined plots
• Add a configuration which field shall be used as time-base (instead of receive time)

[thomasnordquist]

Coming soon, current status as of 9e15e28

[thomasnordquist]

By clicking on the Chart Icon, the chart will be added to a separate panel.

[thomasnordquist]

Released with v0.3.5.