Option to toggle between total and dissolved in plots

For surface water samples there are only total metals. Is it worth adding an option to toggle between total and dissolved metals in plots?

Question - If this app is mostly for `gwc` groundwater chemistry, is adding functionality for surface water samples still within that scope?

The issue names plots and surface-water samples but no files, tests, or entry points. First review how the app currently represents total metals, dissolved metals, and plot options; done requires a decided scope for surface-water support and a defined toggle behavior.

Written by the indexing model from the issue text.