There is no such property for the axes ticks to change individual tick marks -- some possible workarounds (not tried any of these, just the things that come to mind as first-shot attempts to create the effect)
- Set additional tick values at select ticks +/- some small epsilon so the new ticks are just offset from original. Will then have to use ticklabels to label only the ones wanted; can't turn ticklabels on/off on individual level, either.
- Add second axes with above idea -- then can just set the second axis ticklabels to empty
- Draw a heavier line at the desired tick locations with plot() with the desired line width.
The last of the above is probably the simplest...
