Do you have time series for the chart enabled? When you edit a chart, there is a graph settings gear icon that allows you to edit the graph. If you select Time Series it should display.
Also, is this in the Pulse App or are you using the legacy Dashboard. If you are not using Pulse, you should add this app as it gives a lot more graphing flexibility.
https://www.ibm.com/docs/en/qradar-common?topic=app-time-series-charts-in-qradar-pulse
#QRadar#Support#SupportMigration