By evaluating the min and max value as Klaus suggested should usually do the job. Unfortunatly your signal can be interpreted as two sinewaves ("two channels") with an offset.
I'm more curious why your signal doesn't looks as it should, namely a sinewave. What's the signal frequency? I think the maximum arduino (uno) ADC sample frequency is 125 kHz. Do you acquire a whole array (set of samples) at once befor they are sent to the PC?
greets
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Ok, by using analogRead() the arduino ADC sample rate is even lower:
On ATmega based boards (UNO, Nano, Mini, Mega), it takes about 100 microseconds (0.0001 s) to read an analog input, so the maximum reading rate is about 10,000 times a second.