Add way for Server based data in charts

[mitra42]

Want to be able to see old data

• Server: store data in file  #70
• UI to request data from server - as button on graph
• Way to access old data from server (maybe each poll is a day's ?)
• Arrow to request a day's data - each push gets previous day
  • should store last date collected on each dataset and Graph (maybe as millisecs number) of last date
  • When adding new dataset (via button or later from graph itself) should add any back data needed to match the Graph
  • As replace first dataset - using state.data = .. won't work on its own, as graph's dataset needs to point to this new data
  • After first - do push (or unshift?) onto front of data array
  • new data on each dataset should still append
  • Need conversion function for each dataset - prob on place it came from (e.g. MqttBar)
    Completion
• clean up old CSV in imports and importmap
• make something like GETcsv that combines the fetch and parse also GETyaml