RGraph: how to change background color?

I have a web page with 3 rgraph dynamically updated Line charts. I use ajax call for retrieving data from web service. When ajax call completed with error I want to change the chart background color to 'gray'. And when call is successful to change this color to 'black'.

How to change chart background color dynamically?

I use this code

if(error_occured)
 obj_Memory.Set('background.color', '#B6B6B4');
else
 obj_Memory.Set('background.color', 'black');

But it does not change the color.

I modified rgraph sample:

http://ift.tt/1CN7ESs

Added button and simple function:

    var error_occured = false;

    document.getElementById("toggleButton1").onclick = function (e) {
        if (error_occured)
            obj.Set('background.color', '#B6B6B4');
        else
            obj.Set('background.color', 'black');
        RG.redraw();
        error_occured = !error_occured;
    }

It does not change the chart background