Default chart does not always match first series label

[WrathfulSpatula]

For example, see https://metriq.info/Task/141.

[WrathfulSpatula]

To clarify, for unitaryHACK: when tasks have multiple "chart metric" options, the front end app is supposed to start the view on the metric with the most data points. However, while this happens, the "chart metric" option doesn't match the metric plotted, sometimes. Please make the "chart metric" field match the plotted task chart, when landing on the view.