Don't know if it is possible.... but it would be a nice enhancement if tooltips could be utilized to accurately display time & value in the graph.
The below screenshot is from one of my Node-red mobile dashboards, and by tapping the graph line, the tooltip displays the time and value.
Relying upon the emoncms app's normal y-axis scale, we can only estimate the value - somewhere between 2000W and 3000W, whereas with a tooltip we would be able to determine the exact value at any given time.