You may have missed the idea. The squiggle would not represent numeric data - the drawing of the squiggle would. The movement across the x/y of the touchpad would have created that numeric data.
With that in mind, there are numerous ways to accomplish this. One is simply to have the size of the increase/decrease depend upon the X position and either position or velocity of the Y position. For example at a low positioning of Y and increase in X, there would be an increase in the lower digits. Increasing Y (while still moving along the X axis) would create an exponential increase in the upper digits.
I should have had something to demonstrate before mentioning this, but I just now made a simple application (Windows) giving an idea:
2timv.com/misc/NumChng.zip ( there is a windows executable within the archive not a maple worksheet )
That application will require the installation of the .NET 2.0 environment to run. If you don't have it and really want to run it, I've created an installation source that installs the .NET environment as well as the application at the link below:
I designated specific regions in an attempt to simplify the example. The rate of increase within any region (not in my example) could be fine tuned by simply holding down a particular keyboard key, so the rate of increase, even at the smallest level, could just creep along. Keep in mind that the number shown could just as easily represent 98.765932, so the rate of increase for the lowest particular digit of interest could require a great deal of movement anyhow. The decimal point location could be placed when finished, so during construction it could be interpretted however you please.
Rather than the method shown in the example, I think a better model would be to draw a plot as the data is created, and then the movement over the pad in a reverse direction could just as easily be made to automatically backstep (undraw the plot) to decrease the value of the numeric data.
The sample is just thrown together, only about half works, is very sloppy, etc, but should be enough to give an idea of what I'm thinking. If anyone is interested in any of this, I can perhaps make a better example and add it to my blog rather than take up too much space within this one.