Making it clear to user that Value=0 or Error: Unable to fetch value

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.