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.