the categoryX axis is for if your data should all be equidistant. Each point is the same distance from the previous as the next. It does not insert missing values. You can use it with Dates, but they are basically just being treated as strings.

If you want to treat the dates as dates, then you should use the categoryDateTimeX axis or the numericX axis. If you use the numericXAxis you would be converting your dates to numbers to bind along that axis, and you can set the minimumValue, maximumValue and interval to set up the grid line and label display. And you would probable want to specify a formatLabel method to convert the number used back into a date time for displaying in the labels.

If using the categoryDateTimeX axis, then you need to also set a dateTimeMemberPath to the location of the date column, which should be a JS Date, and you can set JS Dates to be the minimumValue, maximumValue, and interval as the numeric timespan that you want to divide up the time range into. You would also want to specify a formatLabel method here to indicate what date time format you want to use with the labels. Here is an example of using the categoryDateTimeX axis:

$(function () {
            
            var currData = [{
                Date: new Date(2011, 1, 1),
                Value: 1
            }, {
                Date: new Date(2011, 1, 2),
                Value: 2
            }, {
                Date: new Date(2011, 1, 4),
                Value: 3
            }, {
                Date: new Date(2011, 1, 5),
                Value: 4
            }, {
                Date: new Date(2011, 1, 8),
                Value: 5
            }];


            $("#container").igDataChart({
                width: "500px",
                height: "500px",
                dataSource: currData,
                axes: [{
                    name: "xAxis",
                    type: "categoryDateTimeX",
                    formatLabel: function (item) {
                        var ret = '';
                        ret += item.getMonth() + 1;
                        ret += "/";
                        ret += item.getDate();
                        ret += "/";
                        ret += item.getFullYear();

                        return ret;
                    },
                    dateTimeMemberPath: "Date",
                    labelAngle: 45,
                    labelExtent: 50,
                    markerType: "circle",
					interval: new Date(2011, 1, 4).getTime() - new Date(2011, 1, 2).getTime()
                },
                {
                    name: "yAxis",
                    type: "numericY"
                }],
                series: [{
                    name: "series1",
                    title: "Test Series",
                    type: "line",
                    xAxis: "xAxis",
                    yAxis: "yAxis",
                    valueMemberPath: "Value"
                }],
                horizontalZoomable: true,
                verticalZoomable: true,
                windowResponse: "immediate",
                overviewPlusDetailPaneVisibility: "visible"
            });

            //            $("#container").bind("igdatachartrefreshcompleted", function () {
            //                    alert("here!");
            //                
            //            });
        });