You need to read the manual of your flowmeter and determine how the actual flow rate relates to the frequency of your signal. Most sensors make it a very easy linear relationship. Then just do the calculation and display your result in an numeric indicator. No need for a graph unless you like looking at a square wave. Although you might want to chart the numeric value so you have a history of the flow rate.
EDIT: There is a VI called "Extract Single Tone Information.vi" that can read the square wave and return its frequency.
LabVIEW Pro Dev & Measurement Studio Pro (VS Pro) 2019 - Unfortunately now moving back to C#, .NET, Python due to forced change to subscription model by NI. 8^{