If I understand you, it's basically an initialization issue. You see a waveform graph can plot several different datatypes: 1D array, 2D array, a cluster containing a x0, delta x and an array of values, and an array of the same clusters (to name but a few). The problem is that while wiring each of thes dataypes to the graph terminal automatically recasts to plot the new datatype, the same thing doesn't happen with a property node.
To fix the problem, momentarily wire data of the same type as you want to send to the property node to the graph's terminal. This action "reconfigures" the graph and you can now wire the data to the property node. In fact depending upon the datatype you select, you may notice the property node change color when you connect the new dataype.
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