Add ability to set custom legend for an expression

[nitzanharel]

For example - a query from Prometheus can return with labels/properties. You can select a specific label/property by setting {{label}} in the legend field.
When you create an expression that is dependent on a query (e.g. $A * 2) - you get the entire legend (like setting the legend to "Auto") and you can't select a specific label/property

[wizardfiction]

Weirdly, sometimes my expression gets the appropriate name, and other times it doesn't.

Example:

$A / ($B * $C), this gives me all of the data from $A along with the name I assigned to my expression, when I just want the expression name.

When I do something like $A / $B instead, I get the correct name without all the extra stuff.

Having a hard time understanding why this is the case

[B3DTech]

You can use math directly in a prometheus query:
(homeassistant_sensor_temperature_celsius{entity="sensor.main_floor_temperature"} * 1.8) + 32

[samlaf]

+1. This would be super useful to have.

[libbkmz]

In case of queries from the same datasource that would work. However, In my case, I have prometheus and graphite. How can it be possible to override the legend name for math expressions in grafana?

[r10r]

I also have several expressions with a long legend in a tiny font that are unreadable . What about using the expression name in the legend?

[itsmylife]

Thank you for submitting a feature request. We’ve reviewed your request and have decided not to implement this feature for the following reasons:

• Field override feature can be used to achieve the same result. Please check the documentation https://grafana.com/docs/grafana/latest/panels-visualizations/configure-overrides/

Even when we make the decision not to pursue a feature, the submission encourages conversations and ideas within our development team to continue making improvements, so please keep sharing!

[jdheyburn]

@itsmylife It would be helpful if you could provide an example. I only see Standard options > Display name to change the name of a series from an expression, and this does not support labels - setting {{LABEL_NAME}} prints it out literally.

[itsmylife]

@jdheyburn Please find the documentation about Display Name
https://grafana.com/docs/grafana/latest/panels-visualizations/configure-standard-options/#display-name

Example ${__field.labels.<the_label_name_inside_the_field>}

[alicederyn]

In case anyone else hits this and wonders what on earth is being talked about, you want something like this:

On the graph properties on the right, switch from "all" to "overrides" tab, then click "add field override", and select "fields returned by query". You can then select the query label for the expression you want (in this case, my expression was the fifth one, so called "E"). Then click "Add override property" and select "Display name", then type the label you want (probably not "foo"!).

This is of course hard to find, hard to maintain, and deeply surprising. Overrides should obviously be listed front-and-center with the expression, not buried off to the right somewhere. But that's how to do it as the UI is currently laid out.

[jaymzh]

and you can't use variables in it :( So if you set it to "p99 {{some_label}}" it doesn't work.