Line chart: x axis data column type is date but i want the x axis to be discrete

[Amarjit Prasad]

I am using a google chart where x axis data column type is date. But i want the x axis to be discrete. Is there any way to accomplish this.
When I changed the  'type' option of the axis to be 'category' then i am unable to see trendlines
here is my code

google.load('visualization', '1.1', {packages: ['corechart']});
google.setOnLoadCallback(drawChart);

function drawChart() {

var revenueChart_93;    

var data = new google.visualization.DataTable();

   data.addColumn('date', 'Month');   
   data.addColumn('number', 'Predicted Grade');      
   data.addColumn('number', 'Working Grade');
   data.addColumn('number', 'Estimated Grade');


data.addRow([new Date(2015, 2, 12),  1.0,  1.0,  1.0 ]);
data.addRow([new Date(2015, 5, 12), 4.5, 5.0, 6.5]);
data.addRow([new Date(2015, 6, 1), 5.0, 6.0, 4.5]);
data.addRow([new Date(2015, 11, 1), 6.5, 7.0, 5.0]);


var formatter = new google.visualization.NumberFormat({
    fractionDigits: 2,     
});
formatter.format(data, 1);
var dateFormatter = new google.visualization.NumberFormat({
    pattern: 'yyyy MMM'
});
dateFormatter.format(data, 0);
var chartHeight = 550;
var chartWidth = 750;
var chartOptions = {
    trendlines: {
        1: {
            color: 'yellow',
    //tooltip: false
        }
    },
    title: 'Progress Report',
    isStacked: true,
    width: chartWidth,
    height: chartHeight,      
    hAxis: {
        title: 'Year',
        slantedText: true,
        slantedTextAngle: 45,
        textStyle: {
            fontSize: 10
        },
        format: 'MMM yyyy', 
    },
    vAxis:{title: "Grade",
            minValue: 1,
            maxValue: 10, 
            gridlines: {
            count: 19
            },
            baselineColor: 'none',


            },       
    series: {
        2: { lineDashStyle: [2, 2] },
         },
 legend: { position: 'right', alignment: 'start' },

};

revenueChart_93 = new google.visualization.LineChart(document.getElementById('progress'));
revenueChart_93.draw(data, chartOptions);
}

[Daniel LaLiberte]

Hi Amarjit,

The trendline feature currently requires that the domain axis data be continuous.  Typically discrete values are not ordered meaningfully, so trendlines based on discrete values would not be meaningful.  

Is there a reason you want to use discrete values instead of dates?  You can also control the format of the tick values and tooltip values, if that is what you really want. You would say type: 'string' by the way.  I'm not sure what happens if you just provide Date values for your string type column (depends on how you declare you're DataTable), but the effect would be that non-uniformly distributed dates would end up being uniformly distributed on a discrete axis.  If you skip weekends, for example, a trendline, assuming it worked, would not know about it.

What you probably should do, if you want each successive date to be used like a discrete value, is change your dates to successive integers and also provide the formatted representation of each integer as the corresponding date converted to a string.  Like so:

var data = new google.visualization.

DataTable();

   data.addColumn('number', 'Month');   
   data.addColumn('number', 'Predicted Grade');      
   data.addColumn('number', 'Working Grade');
   data.addColumn('number', 'Estimated Grade');

data.addRow([{v: 0, f: (new Date(2015, 2, 12)).toString(),  1.0,  1.0,  1.0 ]);
data.addRow([{v: 1, f: (new Date(2015, 5, 12)).toString(), 4.5, 5.0, 6.5]);
data.addRow([{v: 2, f: (new Date(2015, 6, 1)).toString(), 5.0, 6.0, 4.5]);
data.addRow([{v: 3, f: (new Date(2015, 11, 1)).toString(), 6.5, 7.0, 5.0]);

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[Daniel LaLiberte]

Regarding setting the 'type' of the hAxis, I had missed Sergey's suggestion (at https://github.com/google/google-visualization-issues/issues/1986) to set that to type: 'category', which does treat the individual date values as discrete values.  The fact that trendlines feature doesn't work with discrete values is a separate issue that I tried to address with my suggestion.

[Amarjit Prasad]

 Hi  Daniel,

               Thanks for suggestion. Yes I got your point.