First, A graph does not store any values so if you simply clear the array you are wiring into the graph it will also clear it. A chart actually stores values you run into it thats why it actually needs to be cleared. Additionally, this is why you can feed a chart a single value whereas a graph requires as array.
Alternatively, but somewhat unnesessarily, a graph also be cleared by going on your block diagram and right clicking on the graph icon Create>>Property Node. Next, click on 'Visible' and select 'Value'. Right click on 'Value' and goto Create>>Constant. This will wire and empty array to the value property which will clear the graph. Remember, if you put this is your main loop it will clear the graph on every iteration.
