Here's the same diagram in a different style:

Here's the C code that I use for my line drawing function, using the cairo API.

void
crazyLine( cairo_t* ctx, double fromX, double fromY, double toX, double toY)
{
    // Crazyline. By Steve Hanov, 2008
    // Released to the public domain.

    // The idea is to draw a curve, setting two control points at random 
    // close to each side of the line. The longer the line, the sloppier it's drawn.
    double control1x, control1y;
    double control2x, control2y;

    // calculate the length of the line.
    double length = sqrt( (toX-fromX)*(toX-fromX) + (toY-fromY)*(toY-fromY));
    
    // This offset determines how sloppy the line is drawn. It depends on the 
    // length, but maxes out at 20.
    double offset = length/20;
    if ( offset > 20 ) offset = 20;

    // Overshoot the destination a little, as one might if drawing with a pen.
    toX += ((double)rand()/RAND_MAX)*offset/4;
    toY += ((double)rand()/RAND_MAX)*offset/4;

    double t1X = fromX, t1Y = fromY;
    double t2X = toX, t2Y = toY;

    // t1 and t2 are coordinates of a line shifted under or to the right of 
    // our original.
    t1X += offset;
    t2X += offset;
    t1Y += offset;
    t2Y += offset;

    // create a control point at random along our shifted line.
    double r = (double)rand()/RAND_MAX;
    control1X = t1Y + r * (t2X-t1X);
    control1Y = t1Y + r * (t2Y-t1Y);

    // now make t1 and t2 the coordinates of our line shifted above 
    // and to the left of the original.

    t1X = fromX - offset;
    t2X = toX - offset;
    t1Y = fromY - offset;
    t2Y = toY - offset;

    // create a second control point at random along the shifted line.
    r = (double)rand()/RAND_MAX;
    control2X = t1X + r * (t2X-t1X);
    control2Y = t1Y + r * (t2Y-t1Y);

    // draw the line!
    cairo_move_to( _ctx, fromX, fromY );
    cairo_curve_to( _ctx, control1X, control1Y, control2X, control2Y, toX, toY );
}