The exact code depends on your programming language and the type of x-coordinates. If you are using a label based x-axis (the axis is configured using Axis.setLabels), the x-coordinate is the array index of the label, not the label itself. If you are using a true date/time axis (the x-coordinates are provided using Layer.setXData). the x-coordinate is the date/time. In certain programming languages, the date/time is an object (such as System.DateTime object in .NET and java.util.Date in Java), while Axis.addZone expects floating point values. In this case, you can use Chart.CTime to convert the date/time object to a double precision value to be used for Axis.addZone.