The adc (analog digital converter) MCP3008 is implemented in OpenPlotter.
It does read voltage between 0 - 3.3 V. It is has a 10 bit resolution (0-1023).
Many signals aren't linear. To get a nearly linear result you can use the value setting.
The medium level in a tank on boats isn't linear to the height the sensor measures. To calibrate it empty it and insert the ADC value for 0%. fill in 10% of max volume and insert ADC value and 10.0. Go on until the tank is full.
Look at the graph you created by clicking on graph