New canvas clipping functions

Written by Richard, on 28th March 2022

Following on from the previous dual-color Line chart demo here's another example of that sort of chart. The difference with this one is that it uses two new functions that have been added to RGraph: RGraph.ClipTo.start and RGraph.clipTo.end These functions will be available from version 6.07 and make clipping the canvas easier. The documentation for them will be part of the API docs (it will only be there after version 6.07 has been released).

The beforedraw and draw RGraph events are still used to set up and remove clipping, but the RGraph functions make doing this far clearer and easier to understand.

View the example on codepen.io[icon]


    // This is the first chart and draws the green,
    // top half of the chart
    line1 = new RGraph.Line({
        id: 'cvs',
        data: data = [
            4,8,6,3,2,4,-5,-4,5,6,-1,-9,1,2,-3,-5,-7,-4,-2,-2,6,2,3,4,
            4,8,6,2,3,5,1,4,9,5,6,4,3,8,7,5,6,4,5,1,2,3,4,6
        ],
        options: {
            textSize: 15,
            xaxisPosition: 'center',
            yaxis: false,
            backgroundGridHlinesCount: 10,
            backgroundGridVlines: false,
            backgroundGridBorder: false,
            shadow: false,
            linewidth: 0.5,
            colors: ['green'],
            filled: true,
            filledColors:['rgba(0,255,128,0.25)'],
            marginInner: 15,
            spline: true,
            xaxisLabels: ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec'],
            xaxisTickmarksCount: 12
        }
    }).on('beforedraw', function (obj)
    {
        // This is the new function that makes clipping the
        // canvas much easier. It can take a multitude of different
        // arguments. In this case, it simply clips to the top half
        // of the canvas (and then the other chart clips to the
        // bottom half). You can read about all of the different
        // ways that you can call this function in the API documentation
        // ( https://www.rgraph.net/canvas/api.html#functions.clipping )
        // and there's four examples that use it in the demos in
        // the download archive.
        RGraph.clipTo.start(obj, 'tophalf');
    
    }).on('draw', function (obj)
    {
        // This complements the above RGraph.clip.start() function and
        // ends clipping - returning the canvas to a neutral state.
        RGraph.clipTo.end(obj);
    }).trace();
    
    
    
    // And this is the second, red Line chart that's shown on the
    // lower half of the chart.
    line2 = new RGraph.Line({
        id: 'cvs',
        data: line1.data,
        
        // Set the options for the second chart. The ES6 "spread" operator (...)
        // is used here to reuse the options that were set in the first
        // object. Further options are then set in order to distinguish
        // it from the first chart.
        options: {
            ...line1.properties,
            colors: ['red'],
            filledColors: ['rgba(255,128,128,0.25)'],
            yaxisLabels: false,
            xaxisLabels: ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec']
        }
    }).on('beforedraw', function (obj)
    {
        // Start clipping the canvas - this time to the lower half of
        // the canvas.
        RGraph.clipTo.start(obj, 'bottomhalf');
    
    }).on('draw', function (obj)
    {
        // End clipping
        RGraph.clipTo.end(obj);
    }).trace();