joining dots of a scatter plot and create a line

Question

How is possible to join dots of a scatter plot after plotting, and make a line from a dotted plot?

Answer 1

I'm guessing you generated a scatter plot from x and y coordinates by,

plot(x,y,'.');

Join them with

plot(x,y,'.');
hold on;
plot(x,y,'-');

Or in one command

plot(x,y,'.-');

Is this what you wanted?

Answer 2

If you have an existing plot as a scatter plot, you cannot simply just join the dots without knowing which points are connected to which others.

If you know the order/connectivity of the points, then you could simply have used the plot function to do that in the first place. The call

plot(x,y,'-')

will connect the dots with straight line segments. If you wish to use a marker symbol at each point along the line, then you can add one of the markers that plot allows, as this:

plot(x,y,'o-')

You can get a list of the allowed markers from

help plot

If you have used scatter on a set of points, and now wish to overlay a line connecting the points, then use the hold function to force matlab to plot on top of the scatter plot. For example,

scatter(x,y)
hold on
plot(x,y,'-')
hold off

Again, any of these variations require you to know the connectivity between the points. There are some schemes that can sometimes work to recover that connectivity from a list of isolated points. One of these methods is called CRUST, often used for 3-d surface reconstruction. I found many references by a simple search for "crust algorithm".

Answer 3

If you have a scatterplot (made with the scatter function I suspect) and for some reason don't want to redraw it with plot, here is what you can do to connect the dots:

h = findobj(gca,'type','hggroup');
hold on
for k=1:numel(h)
    x = get(h(k),'xdata');
    y = get(h(k),'ydata');
    plot(x,y,'-')
end
hold off

The dots will be connected by their original order. If you want you can sort the data before plot, for example by x:

[x,ind] = sort(x);
y = y(ind);