Linear regression - how to plot at specified locations

I have a vector of basis/spline locations that I want to use to plot a piecewise linear regression to.

I have

y_i=B*beta + error.

B is a design matrix that calculates the time point in the data minus the knot location (each theta) when the differene is positive (x_i-theta_i)_+ , for each n data points.

How do I actually proram this in MATLAB? I want to plot y against x, where the linear regression changes at each theta?

Any help in the right direction would be much appreciated.