Electronic – how to use the smartphone as oscilloscope

The library was written and tested in:

Arduino Alpha 0022

Have you tried this?

#if defined(ARDUINO) && ARDUINO >= 100
  #include "Arduino.h"
#else
  #include "WProgram.h" #endif 

A oscilloscope plots voltage as a function of time, so your display is reasonable as you show it. However, the term "time base" is meaningless to indicate the X axis scale. What you want is "s/div" (or ms/div or µs/div). This is independent of the sample rate, although there is little point using more than a few pixels per sample.

The sample times you mention are very slow for ordinary oscilloscopes. Some signals will be reasonably visible at those rates, but most things you encounter will not be.

I would probably figure out what the fastest sample rate is that you can support, then always sample at that rate. If the application indicates it does not need samples that fast, then you can merge multiple samples into one before sending over the network. In that case you don't want to do traditional decimation, which seeks to eliminate frequencies that alias. Instead, for each data point send the min and max A/D samples covered by that data point. Each data point should then be shown to vertically cover that min/max range. If the user selects a slow sample rate and a faster signal is being sampled but it is still within the capability of the A/D and the underlying fast sample rate, then the display will be a horizontal bar with vertical width showing the signal peaks. That is a much better display than something that aliases.