Sorry, but I thought the fractal dimension (assuming Hausdorff's) doesn't make sense with mixed dimensions (such as time/space or time/amplitude) and _especially_ doesn't make sense on a noncontinuous domain, as there has to be ever-smaller details for the Hausdorff's to be noninteger.
Well, there are some definitions that allow you to calculate it on any curve that is sampled. It is used for detection onsets of heart-rhithm irregularities and other event-onsets in noisy environments. What you can do is to (linearly) map the time-amplitude graph to a unit square and look at the shape. There don not need to be fractal patterns for the dimension to be non integer. All it is, is a proportion between different coverings of the shape with ever smaller squares or circles. If the domain is sampled you can take the single samples as the smallest covering and compare that with the unit square. (which is why you map it to the unit square in the first place. At least that is one method.
Basically it is looking at the shape of the graph and trying to find usefull information.
Gerard