Code Produces Error When Trying to Operate a Particular Userform after Another

I have a program that plots voltage and current waveforms and digital inputs and outputs of power system relays. I am running into a problem that if I have plotted a digital plot, when I try to plot the waveforms I get an error. The line of code that highlights is the line of code that sets the X axis values for the plot. The only time this happens is when a waveform is attempted while a digital exists. You can plot waveforms, then go back and run the waveforms macro again, and everything will work fine. You can also do multiple digital plots in a row. Also, you can plot a waveform first and then plot a digital without flaw. The only issue is trying to plot a waveform after a digital. Please see the attached sample spreadsheet so that you can reproduce what I'm seeing. Just run the macros in whatever order so that you can get a better feel for what I am talking about. Please let me know if further clarification is needed. Does anyone know what the issue may be? My troubleshooting techniques are very novice with VBA...Thanks in advance for the help!