See yesterday's news...<how-can-i-change-the-pspectrum-time-units> and, while it doesn't look related from the title, it's got the way to do whatever you wish with the axes <heatmap-axis-labels-printing-vertically>.
The stackedplot uses the same form of the graphics object as does heatmap; the Axes object is the last element of the hidden NodeChildren property (altho there are only two items in the array, not three).
Follow the examples there to retrieve that handle and go from there...
