"I have a plot" does not explain, in which format this "plot" is. Printed on paper? As .fig file? In a PDF? In the current figure?
Let's assume, that the plot is the GCA and it contains one line only:
LineH = get(gca, 'children');
Value = get(LineH, 'YData');
Time = get(LineH, 'XData);
[maxValue, maxIndex] = max(Value);
maxTime = Time(maxIndex);
If your plot is available in another form, please specify it exactly. Be aware, that the above method replies one value only, even if the maximum value appears several times in the data.
