Graphical interpretation of partial derivatives of a function of two variables

The interactive demo below shows the surface defined by the function $z=f(x,y)=\sin(xy)$ together with the cross-sectional curves through the surface parallel to the $x$ and $y$ axes and based at the base point $(x,y)=(a,b)$ . Use the slider controls to set the values of $a$ and $b$ and observe the form of the cross-sectional curves and values of the partial derivatives.

f(x,y)=sin(x*y)
xlim = 5*pi/4 # limit for the x coordinate
ylim = 5*pi/4 # limit for the y coordinate

@interact
def demo(a = slider(-xlim,xlim,2*xlim/100,0), b = slider(-ylim,ylim,2*ylim/100,0)):
    p=plot3d(f(x,y), (x, -xlim, xlim), (y, -ylim, ylim), opacity=0.8)
    xslice=parametric_plot3d([x,b,f(x,b)],(x,-xlim,xlim),color='red', thickness=8)
    yslice=parametric_plot3d([a,y,f(a,y)],(y,-ylim,ylim),color='green', thickness=8)

    pnt=point((a,b,f(a,b)), color='black',size=10)

    show('The surface defined by $z = f(x,y)$ with cross-sectional curves, from the base point $(a,b)=('+str(n(a,digits=3))+','+str(n(b,digits=3))+')$'+ ' in the $x$ and $y$ directions.')
    show(p+xslice+yslice+pnt, spin=2.5)

    fx=f.diff(x)
    fy=f.diff(y)

    p=plot(f(x,b),(x,-xlim,xlim),color='red', thickness=6, axes_labels=['$x$','$z$'])
    q=plot(f(a,y),(y,-ylim,ylim),color='green', thickness=6,  axes_labels=['$y$','$z$'])

    show('The derivative $f_x(a,b)\\approx'+str(n(fx(a,b),digits=3))+'$.')
    show(p+point((a,f(a,b)), color='black', size=150,zorder=5))
    show('The derivative $f_y(a,b)\\approx'+str(n(fy(a,b),digits=3))+'$.')
    show(q+point((b,f(a,b)), color='black', size=100,zorder=5))

Interact: please open in CoCalc