Analysis time series view: long task names clutter up dropdown

[MetRonnie]

I'm not sure displaying the selected tasks as chips in the v-autocomplete is working out. And the selected tasks are already shown in the chart legend, so why not just display the number of selected tasks in the v-autocomplete instead of the tasks themselves?

You can do this by providing the #selection slot: https://vuetifyjs.com/en/api/v-autocomplete/#slots-selection

Originally posted by @MetRonnie in https://github.com/cylc/cylc-ui/pull/1510#discussion_r1579686754