How to mark a point in a MATLAB plot?

Answer 1

One way to do this is to use the GINPUT function to graphically select a point using the mouse. Assuming the data you plotted is stored in a variable data, the following code should do the sort of thing you want.

set(gca,'XLimMode','manual','YLimMode','manual');  % Fix axes limits
hold on;
[x,y] = ginput(1);  % Select a point with the mouse
x = round(x);       % Round x to nearest integer value
y = data(x);        % Get y data of intersection
plot([x x],get(gca,'YLim'),'k--');  % Plot dashed line
plot(x,y,'r*');     % Mark intersection with red asterisk
disp('Intersection coordinates:');
disp([x y]);        % Display the intersection point

The above assumes that the x-values of the graph are just indices into the array of data you're plotting, which appears to be the case from the graph you show above.

Answer 2

Try something like:

x = 1020;

% plot a vertical line
ylimits = get(gca, 'YLim');
hold on;
plot([x x], ylimits, 'k');

% mark the intersection with the plot
plot(x, data(x), 'ro');
annot = sprintf('Intersection: x=%f, y=%f', x, data(x));
text(x, data(x), annot);

The code is not tested and assumes that your figure is the current one, the plotted data is stored in the array "data" and that the original plot was done without specifying an extra x-vector.