Plotting data from Arduino and using the serial monitor in parallel
Is using the Serial Monitor a requirement, or are you just looking for a good way to get and plot the data?  If the latter, you may be interested in the open-source DaqPort sketch. https://www.daqarta.com/dw_rraa.htm  This was created to use the Arduino as a peripheral for data acquisition, as opposed to the more typical case of the Arduino being the central device.  With DaqPort you can program in any language you like on the host computer, and use it to plot data from the Arduino. The Daqarta software package that includes DaqPort has several examples that use Daqarta's macro language to implement a multi-channel chart recorder, high-speed multi-function oscilloscope, timers, signal generator, etc.  You can use these for free, forever, even after Daqarta's 30-day trial ends.  Since Daqarta already has plotting, you may find it simpler to use than rolling your own, perhaps by modifying one of the examples.