How is Charting Different Than Plotting?
Graphs and charts differ in the way they display and update data. VIs with a graph usually collect the data in an array and then plot the data to the graph. This process is similar to a spreadsheet that first stores the data then generates a plot of it. When the data is plotted, the graph discards the previously plotted data and displays only the new data.
In contrast, a chart appends new data points to those points already in the display to create a history or a buffer. On a chart, you can see the current reading or measurement in context with data previously acquired. When more data points are added than can be displayed on the chart, the chart scrolls so that new points are added to the right side of the chart while old points disappear to the left.
What Types of Data Can I Chart?
You can chart numeric data contained in arrays and analog waveforms. By default, the chart data type is an array of doubles. You can change the data type on the diagram.
The waveform data type carries the start time (t0), delta t (dt), and y-values (Y) of a waveform. When you wire waveform data to a chart, the chart automatically plots a waveform based on the start time, delta t, and y-values of the waveform. A waveform that specifies t0 and a single-element Y array is useful for plotting data that is not evenly sampled because each data point has its own time stamp.
For data contained in arrays, the chart displays a general-purpose Cartesian graph that charts any set of points, evenly sampled or not.
The following table describes how different types of array data are charted.
|1D array of numeric values
The array is a single plot.
- X-value: Array index
- Y-value: Array element
|1D array of waveforms
||Each waveform is a separate plot.
|2D array of numeric values
||Each column of the array is a separate plot.
How Do I Define the Chart History Length?
You use the Chart History dialog box to configure the size of the buffer. The default chart history length for a waveform chart is 1,024 data points. The frequency at which you send data to the chart determines how often the chart redraws.