Stop or destroy a jQuery plugin

I have a push menu and it has an animated gradient but I only want it to animated/play when the menu is open, obviously for performance reasons.

My gradient plugin code is as follows:

// Animated Gradient
var colors = new Array(
   [62,35,255],
   [60,255,60],
   [255,35,98],
   [45,175,230],
   [255,0,255],
   [255,128,0]);

var step = 0;
var colorIndices = [0,1,2,3];

var gradientSpeed = 0.001;

function updateGradient()
{

if ( $===undefined ) return;

  var c0_0 = colors[colorIndices[0]];
  var c0_1 = colors[colorIndices[1]];
  var c1_0 = colors[colorIndices[2]];
  var c1_1 = colors[colorIndices[3]];

  var istep = 1 - step;
  var r1 = Math.round(istep * c0_0[0] + step * c0_1[0]);
  var g1 = Math.round(istep * c0_0[1] + step * c0_1[1]);
  var b1 = Math.round(istep * c0_0[2] + step * c0_1[2]);
  var color1 = "rgb("+r1+","+g1+","+b1+")";

  var r2 = Math.round(istep * c1_0[0] + step * c1_1[0]);
  var g2 = Math.round(istep * c1_0[1] + step * c1_1[1]);
  var b2 = Math.round(istep * c1_0[2] + step * c1_1[2]);
  var color2 = "rgb("+r2+","+g2+","+b2+")";

  $('#primary-menu').css({
    background: "-webkit-gradient(linear, left top, right bottom, from("+color1+"), to("+color2+"))"}).css({
    background: "-moz-linear-gradient(left, "+color1+" 0%, "+color2+" 100%)"});

step += gradientSpeed;
if ( step >= 1 )
{
    step %= 1;
    colorIndices[0] = colorIndices[1];
    colorIndices[2] = colorIndices[3];

    colorIndices[1] = ( colorIndices[1] + Math.floor( 1 + Math.random() * (colors.length - 1))) % colors.length;
    colorIndices[3] = ( colorIndices[3] + Math.floor( 1 + Math.random() * (colors.length - 1))) % colors.length;

}
}

I have managed to only play the animated gradient when I open the menu but I'm having trouble stopping it when I close the menu, how can I do this?:

showmenu.onclick = function() {

    if ($('nav').hasClass('visible')) {

        // Closed menu code here

    } else {

        // Opened menu code here 

        // Initialise plugin when menu is open
        setInterval(updateGradient,10);

    }

};