I have the following use case:

The user can select one or multiple of Items A/B/C... and be shown a time series graph of var X against t for that Item(s). There is only 1 graph shown, so when multiple items are selected, the frontend overlays them into a single graph.

Sometimes, the value of X can just be 0 for the entire duration for an Item(s).
OR there could be a technical error where the data can't be fetched.

This can lead to a situation where the user sees 0 for the entire duration for Item A, they might think its a technical error, when its really the case that the value is 0.

How do I make this differentiation (Error fetching data VS value=0) clear to the user?
Especially when Item A might be a case of value=0, Item B is an error, and both are shown on the same graph.


The most obvious solution is to not rely on the graph to indicate an error. Instead, if there is an error fetching data, then inform the user of that with a simple error message somewhere near the graph:

There was an error fetching the data.

If it is a case that some data points will error, but some will be valid, then change the message like so:

This graph is only partial representation as some of the data points could not be fetched.

Ideally, if your graphing tools support it, you could colour the erroneous nodes a different colour and let them have a tool tip that states the data could not be fetched.

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