plot discontinuous line?

[DMStevenson]

Hi - I was wondering if anyone has had to plot a discontinuous line. That is, for example, plot points 1,2,3 as a line, then no line between points 3,4, then a line again between points 4,5,6. I have tried various combinations of ShowPoints, and LinePen.Styles, but cannot find a way to do this. I could, of course, use multiple lineSeries, but my data sets I am plotting are *very* large, so this seems unworkable. Has anyone else found a need for this?

[Thaddy]

If I understand you correctly, you want to do a kind of collision detection?
I.e. if a line crosses another then it should not progress drawing any further?

[440bx]

TTBOMK, there is no function (in Windows at least) that will output a number of disconnected line segments.

OTH, even with a large dataset, it takes very little code to implement that.  for instance:

Step 1. MoveTo(InitialPoint)
Step 2. Draw line to end point
Step 3. if there are more segments, advance to next segment, go to step 1.

Either a "for" or "while" loop could implement that.  It looks like it would be less than 10 lines of code.

HTH.

[wp]

Add a NaN ("not a number", defined in math) as y value to the series at an intermediate x point (https://wiki.lazarus.freepascal.org/TAChart_documentation#Skipping_source_items).

Code: Pascal  [Select][+][-]

1. uses
2.   math;
3.  
4. procedure TForm1.FormCreate(Sender: TObject);
5. var
6.   i: Integer;
7. begin
8.   for i := 0 to 10 do
9.     Chart1LineSeries1.AddXY(i, random);
10.   Chart1Lineseries1.AddXY(10.5, NaN);
11.   for i := 11 to 20 do
12.     Chart1lineseries1.AddXY(i, random + 1);
13. end;

But be warned: It is not allowed to do any calculations with NaN, even "if x = NaN" will crash your application (use "if IsNaN(x)" instead). Usage of Nan is in TAChart for more than 10 years, and I hope that all such occurrencies now have been eliminated, but you never can tell...

[DMStevenson]

Thank you wp! This will solve my problem exactly, and also thanks for the reference in the lit. (did not find that passage). I need this as data output from one of our instruments occasionally contains 'impossible' values (usually 0), and yet we need to graph the results - skipping the untenable values. This will work beautifully!

[VisualLab]

Continuing the topic of discontinuity of functions - is it possible to draw graphs of functions (as one series) using TAChart:

• having a jump discontinuity,
• having a point of discontinuity,
• having asymptotes.

I mean examples such as those presented in this presentation.

[wp]

Yes, of course. Since a few version you can even do it in design mode:

• Add a TExpressionSeries to the chart
• Define the ranges for the x and y axes: Chart.LeftAxis.Range.XMin = -10, .XMax = 10, .UseMin = true, .UseMax = 10. The same with the BottomAxis. (You can also enter this information in the Extent property of the ExpressionSeries, but this is more work when you want to add serveral series).
• Type the math expression into the property Expression, e.g. 1/x^2 --> You see the function in the chart!
• But at x = 0 there is a vertical line representing the discontinuity: In property Domain tell the parser where the function is not defined - enter "x <> 0" (without quotes) - the vertical line disappears.
• To see the formula in the legend, set Chart.Legend.Visible = true. Enter the function expression in the series' Title. You can get proper superscript for the exponent by using a HTML expression: "y = 1/x²" (no quotes). In order to activate HTML, set Chart.Legend.TextFormat to tfHTML.

But be careful: save often. As I noticed while preparing this, the parser is not very tolerant regarding typos, and this will easily freeze the IDE when you do this at designtime.

[VisualLab]

wp, thanks for the tips on creating expression-based function graphs.

It's nice that there is a description of how to use the TAChart parser: How To Use TFPExpressionParser.