@interact
def f0(fun=x*sin(x^2), mousemove='', click='(0,0)'):
    click = sage_eval(click)
    g = plot(fun, (x,0,5), zorder=0) + point(click, color='red', pointsize=100, zorder=10)
    ymax = g.ymax(); ymin = g.ymin()
    m = fun.derivative(x)(x=click[0])
    b =  fun(x=click[0]) - m*click[0]
    g += plot(m*x + b, (click[0]-1,click[0]+1), color='red', zorder=10)
    def h(p):
        f0.mousemove = p
    def c(p):
        f0(click=p)
    show(g, events={'click':c, 'mousemove':h}, svg=True, gridlines='major', ymin=ymin, ymax=ymax)