This works by lowercasing the value of `label` and, if there are more words separated by spaces, replacing those spaces with an underscore (`_`). The transformed value is then checked against the labels of the time series returned by the Prometheus query. If a time series label is found that is equal to the transformed value, then the label value will be used and rendered in the legend like this:
![legend with label shorthand variable](img/prometheus_dashboard_label_variable_shorthand.png)