How do I add up two scatter points with the same values of x but different values of y?

Question

On a stem plot, how can I add points that have the same values of x but different values of y ?

For example, given the following code:

x = [1 2 3 6 6 4 5];
y = [3 6 1 8 9 4 2];
stem(x,y);

If you plot x , and y , this will be the output:

I want to add up (6,8) and (6,9) so it becomes (6,17) , just like what the image is showing.

How can I achieve this?

Answer 1

Use accumarray with x and y so you can bin or group like entries together that share the same x . Once these values are binned, you can sum all of the values that share the same bin together. As such, we see that for x = 6 , we have y = 8 and y = 9 . accumarray allows you to group multiple y values together that share the same x . Once these values are grouped, you then apply a function to all of the values in the same group to produce a final output for each group. In our case, we want to sum them, so we need to use the sum function:

x = [1 2 3 6 6 4 5];
y = [3 6 1 8 9 4 2];
Z = accumarray(x(:), y(:), [], @sum);
stem(unique(x), Z);
xlim([0 7]);

We use unique on X so that we have no repeats for X when plotting the stem plot. unique also has the behaviour of sorting your x values. Doing x(:) and y(:) is so that you can make your input data either as row or column vectors independently. accumarray accepts only column vectors (or matrices, but we won't go there) and so doing x(:) and y(:) ensures that both inputs are column vectors.

We get:

The above code assumes that x is integer and starting at 1. If it isn't, then use the third output of unique to assign each number a unique ID, then run this through accumarray . When you're done, use the output of accumarray like normal:

[xu,~,id] = unique(x);
Z = accumarray(id, y(:), [], @sum);
stem(xu, Z);