To draw charts, you need to supply data. In ChartDirector, data are supplied as an array of numbers.
In many cases, a data point will be missing. For example, suppose you are drawing a line chart with one data point per hour. It is possible that due to various reasons, some data points could be missing. However, if you are using an enumerated x-axis (see Axis.setLabels), you cannot not simply remove that point, otherwise the remaining points will all be shifted.
To address this kind of issues, ChartDirector supports a special constant called NoValue, which is equal to 1.7E+308. When ChartDirector sees that a data point is NoValue, it will jump over that point.
In a line chart, if a data point is NoValue, ChartDirector will jump over to the next data point. ChartDirector can use an alternative line color when jumping over a NoValue point to visually reflect that there is a missing point. The default alternative line color is Transparent, which means the NoValue point will result in a discontinuous line. You may use LineLayer.setGapColor to change it to other colors, such as using SameAsMainColor (same color as the normal line) or a dash line color.