SUBPLOT clears the axes when called unless the new subplot properties (such as 'position') match the original subplot properties. There are two possible approaches one can take to work around this issue:
1) Use get(gcf,'Children') to obtain the handle to a particular plot (in this example, the leftmost plot).
close all;
clear all;
open test.fig;
%%Obtain handles to all subplots in current figure
h = get(gcf,'Children');
%%Extract subplot position information
for i = 1:3
positions(i,:) = get(h(i),'position'); % extract positions of each subplot
xpositions(i) = positions(i,1);
end
%%Find index of leftmost subplot
[xposition_min leftmost_index] = min(xpositions);
%%Set 'NextPlot' property of leftmost plot to 'add' (analogous to HOLD ON)
set(h(leftmost_index),'NextPlot','add');
%%Plot new data
plot(h(leftmost_index), [30 31 32],[15 20 25], 'bo');
2) When creating future subplots, set the 'Tag' property of the subplot to a memorable name and search for the tag when needed.
%%Create 3 subplots with tags
clear all; clf; clc;
h = subplot(1,3,1);
plot(sin(0:0.1:10));
set(h,'Tag','left');
h = subplot(1,3,2);
plot(tan(0:0.1:10));
set(h,'Tag','center');
h = subplot(1,3,3);
plot(sec(0:0.1:10));
set(h,'Tag','right');
%%Add new data to leftmost plot
h = findobj('Tag','left'); % get handle to object tagged as 'left'
set(h,'NextPlot','add'); % set 'NextPlot' property to 'add'
plot(h, cos(0:0.1:10),'r--'); % plot new data
