I usually use GetData software. while with matlab, I wrote code for this problem below
% Extract data from image
clc;clear
figName = input('Insert the name of you picture(jpg or png)\n','s');
myFig = imread(figName);
figure(11)
imshow(myFig);
xmin = input('min of x:');
xmax = input('max of x:');
ymin = input('min of y:');
ymax = input('max of y:');
nCurve = input('input number of curves: n\n');
title('click left_up corner and right c_down orner in the axis：')
[xcor, ycor] = ginput(2);
Curve = struct([]);
for i = 1:1:nCurve
title(['Please click the points of ',num2str(i),'th curve（end with enter）'])
[Curve(i).x , Curve(i).y]= ginput(100);
Curve(i).x = xmin+(Curve(i).x - xcor(1))./(xcor(2)-xcor(1))*(xmax-xmin);
Curve(i).y = ymin+(Curve(i).y - ycor(2))./(ycor(1)-ycor(2))*(ymax-ymin);
end
figure(12)
Leg = cell(nCurve,1);
color = 'rgbkcmrgbkcmrgbkcm';
marker = '^osd><hvposd><hvposd><hvp';
for i = 1:1:nCurve
plot(Curve(i).x,Curve(i).y, [color(i),'-',marker(i)],...
'markerfacecolor',color(i),'markersize',5)
hold on
Leg{i,1} = ['Line ',num2str(i)];
end
axis([xmin xmax ymin ymax])
legend(Leg);
xlabel('X')
ylabel('Y')
set(gca,'fontsize',15)
save(['Curve',figName,'.mat'],'Curve')
