I think you need to be a bit more precise in what you want: you have data with errors in them, and you want to fit a curve that approximates those points, and then get the slope of the curve?
If you want to fit a spline that goes through all data points (of 1D function), and get the slope one simply way to do it is to use interp1:
POL = interp1(xx,yy,'spline','pp');
dPOL = fnder(POL);
dydx = ppval(dPOL,xi);
where xi are your query points. Something like that.