pchip plot in relation to y axis

I'm trying to plot a curve using pchip but it is filling data points in the wrong direction.

x = Tempdif;
y = Pres;
xq = linspace(-1.5, 2, 200); 
pp = pchip(x, y);
yq = ppval(p, xq);
figure(1)
plot(xq, yq, '-r',x, y, 'ob' )

This is what I want it to look like but smooth

This is what I'm getting

How can I get the red line to be processed horizontally instead of vertically?

UPDATE: Trying the answer by John D'Errico the values on the axes have been flipped

Answer 1

[John D'Errico]

You are fitting the function in the WRONG direction. This NOT a problem of fitting pressure as a function of temp difference. There is no functional relationship there.

The relationship that you have seems to be of the form tempdif(pressure). Thus as pressure changes, the temperature difference changes dependently.

But fine, you can still plot it with the variables on the axes you have chosen.

y = Tempdif;
x = Pres;
xq = linspace(min(x), max(x), 200); 
pp = pchip(x, y);
yq = ppval(pp, xq);
figure(1)
plot(yq, xq, '-r',y, x, 'ob' )
xlabel 'Temperature difference (C)'
ylabel 'Pressure (nPa)'

You did make a good choice to use pchip instead of spline.

[Electric Sheep]

Thanks that's got the red line going in the right direction but somehow is flipping the axes for the data points. It's a plot of atmospheric temperature bias so I'm using the pressure as a representation of height.