Hi Tamara,
The x-axis is time points, which should match the EPI data
(though some matrices, like X.xmat.1D, has censored time
points removed, which is why X.nocensor.xmat.1D is there).
The y-axis is determined by the regressors. For X.stim.xmat.1D,
it contains the actual regressors of interest, most of which are
probably close to unit height, though motion and modulators
do not need to be).
X.sum.xmat.1D is just the sum of regressors in X.stim.xmat.1D.
- rick