specifies the chart gradient
webix.ui({
    view:"chart",
    type:"bar",
    gradient:"falling",
    ...
})
The property is applicable only to the bar, pie charts
In bar charts the property can be defined as:
As a string, the property takes one of predefined values, which are:
webix.ui({
    view:"chart",
    type:"bar",
    gradient:"rising",
    ...
})
Related sample: Chart: XML Dataset
As a function, the property accepts 1 parameter - a canvas gradient object.
You may use the addColorStop(position, color) method to assign the required colors to the gradient object:
webix.ui({
    view:"chart",
    type:"bar",
    gradient:function(gradient){
        gradient.addColorStop(0.0,"#FF0000");
        gradient.addColorStop(0.8,"#FFFF00");
        gradient.addColorStop(1.0,"#00FF22");
    },
    ...
})
Related sample: Color Gradient
In pie charts the property can have only a boolean value.
The true value enables the gradient in the chart:
webix.ui({
    view:"chart",
    type:"pie",
    gradient:true
    ...
})
Related sample: Donut Pie Chart