UI: Add charts

[mitra42]

*Note - working on this on an unpushed branch, please ask if want to collaborate and I'll share it

Need to add charting capability -
Two main use-cases,

• UX running just off MQTT messages, graphed as received
• UX running on back data pulling data from the server.

At first look, chart.js looks like it has what is needed, appears to run client side (good),

• Add chart to UI based on static (fake) data
• Allow adding data from MQTT messages
  • Add a child to mqtt-chart that is a single dataset, and is a subclass of MqttReceiver - it could also be the control for that line.
  • Child should add data points.
  • and what color
  • How does child redraw chart ?
  • Figure a way to add new datasets via UX (e.g. click graph icon next to bar to have on chart)
  • Dont add to graph if already added (i.e. clicked graph icon twice) (L340 opengraph)
  • Why does outer box of Project include only half of chart
  • Fix legend that just says "undefined'
• Start saving data as soon as UI is up - so graph can display later #76
• Figure out how to show a gap in data  #75
• Chart: Y-axis #74
• Charts: Add boolean values #73
• Add way for Server based data in charts #72
• Handle two similar lines - e.g. temperature from different devices. #80

Further

• Allow adding new line to chart from chart itself - (+ button)
• Modify color if two lines same color
• Modify label if two lines same - e.g. Temperature -> ESP8266-A/temperature
• Allow adjusting X axis
• Allow adjusting Y axis - which changes what data shown
• Allow saving configuration and reloading
• Add info to the discovery that suggests what type of line/columns etc
• Consider (but probably not) merging MqttGraphdataset with MqttReceiver, i.e. all receivers remembering data.

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