2049 multi series chart clear on refresh

[colinl]

Description

Fix bugs that caused charts, where multiple series data are passed in each message, to be cleared on refresh.

Related Issue(s)

Closes #2049

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[colinl]

There were two issues

1. Server side, in ui_chart.js, the filter code that removed old data did not allow for the fact that msg._datapoint could contain an array of series.
2. Client side, in UIChart.vue, add(msg) did not correctly handle the array of messages passed from onLoad, if each message contained multiple series.