... based on the selection in first dropdown list. Functionality based
on a new `hidden` CSS class

Create parent `<div>` elements for both dropdown lists. In addition,
attach a barebones event listener to the new dropdown list

Get the max and min values of the heatmap color axis dynamically.
Fix the tooltip text for `null` observation values.

Deactivate datastreams that do not have observations

Formats datastream metadata for use in both linechart and heatmap

Add parameter for datastream metadata to function that draws line chart

Fetch datastream metadata together with the observations

Add parameter for datastream metadata to function that draws heatmap
chart

Fix typo in one of the property names. Perform minor tweaks to function.

There seems to be conflict between the backend server (Node.js &
Express) and our front-end (browser-based) if we want to use module
imports and exports. It is not currently possible to use Node.js /
CommonJS modules. The current solution uses ES6 modules. May need to
look for a better approach.