The way you are accessing pix_value_mat indicates it could be a scalar. If there a single value plotted over the values, then, the output will be blank. It would be good to know that the values or dimensions of TE and pix_value_mat.
You can try this,
plot(TE(1:end),squeeze(pix_value_mat(i,j,k)),'*'); hold on;
The above will confirm, if it is a scalar value that it is accessing in the plot.
Hope this helps.