Added an option to set starting point on X axis.

[kibotrel]

When you want to plot a real math function it is better to at least specify the right X,Y coordinates for a specific point. So by setting the offset on X axis to match what you want to plot, you'll end up with a correct graph.

In this use case I'm assuming you're plotting your y values as follow:

const data = []

for (let x = -25; x < 25; x += 1) {
  const y = someFunction(x)

  data.push(y)
}

As i start computing from x = -25, this option x_begin must be set on -25 as well.

[richardeoin]

Looks good to me, thanks!