[SOLVED] is it possible to have a constant bar type plot?
lazarus / 55-TAChart

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madepabloh · 2012-10-12 17:35

Let me explain what i want to do:

I have a temperature vs date-time plot . I used the calendar dialog to
obtain the dates to plot two vertical constant lines corresponding to
the starting and finish points of the period in which the temperature
remains below 0ºC. For the moment i am working on the visual part, so
instead to have two lines at the beganing and end of this period, i
think it could be more visual to have a bar-like plot. However, it
have two problems for what i want:

1) It is not constant on vertical such as it is the constant line
plot.
2) It is opake, and covers the grid lines of the plot.

So, my question are: is it possible to have something like a constant
line but with the width defined by two x points? and is it possible to
have the bar below the grid of the plot?

Thanks!

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wp · 2012-10-12 19:49

Sorry, I don't quite understand. Maybe you should post a sketch of
what you want to achieve.

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madepabloh · 2012-10-12 20:14

Excuse me, my english is not good enough:

I attach an image showing what i have now and what i would like to
have: a bar-type plot, with infinite limits in Y-axis, and width
(x-axis) defined by two values i provide, with similar behaviour than
Constant line type of plot.

It is more clear in the image (i think).
Thanks!

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wp · 2012-10-12 22:57

Maybe there are better solutions, but if you have special drawing
requirements you can use the OnAfterDraw* events provided by chart. In
this case, I think the correct one is OnAfterDrawBackWall. You may use
the following event handler:

Code: [Select]uses
  LCLIntf;  // for rgb function

procedure TForm1.Chart1AfterDrawBackWall(ASender: TChart; ACanvas:
TCanvas;
  const ARect: TRect);
var
  R: TRect;
  p1, p2: integer;
begin
  p1 := Chart1.XGraphToImage(Chart1ConstantLine1.Position);
  p2 := Chart1.XGraphToImage(Chart1ConstantLine2.Position);
  R := Rect(p1, ARect.Top+1, p2, ARect.Bottom);
  ACanvas.Brush.Style := bsSolid;
  ACanvas.Brush.Color := rgb(220, 230, 210);
  ACanvas.FillRect(R);
end; 

This procedure converts the positions of the two vertical lines to
pixel coordinates. Then it paints a rectangle over the background
before axes, grid, series etc. are drawn. Therefore, the rectangle
does not cover any details of your chart.

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madepabloh · 2012-10-13 06:47

Thanks @WP (one more time),

It works nice. I don´t know if, such as you said, it is the better
solution or not, but it worls perfect!!

So, i will work around this solution to adapt it to my proyect. Thanks
so much!

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madepabloh · 2012-10-13 07:31

I have now a small problem with this solution: when i do zoom or
paning, the painted area goes outside the limits of the plot, crossing
and covering the axis (but not the markers and tittle... Take a look
to the image i post here.

It only occurs in the vertical axis, but not in the horizontal ones.
However, i will also would like to apply this solution to horizontal
constant lines, so probably the problem will be similar but for the
x-axis.

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wp · 2012-10-13 09:36

When you zoom or pan the vertical lines may move out of the chart
area. But in my code the background rectangle is always drawn between
the vertical lines, so it will paint over the left axis if the left
marker is beyond the left axis -- there is no automatic clipping. What
you may do is to check the left edge of the x axis as a limit, this is
Chart1.CurrentExtent.a.x. Therefore:

Code: [Select]uses
  TAChartUtils, math;
var
  ex: TDoubleRect;
// ...
  ex := Chart1.CurrentExtent;
  p1 := Chart1.XGraphToImage(Max(Chart1ConstantLine1.Position,
ex.a.x));
  p2 := Chart1.XGraphToImage(Min(Chart1ConstantLine2.Position,
ex.b.x));

This also takes care of the right edge of the chart.

Not tested, but should work as long as you don't use
transformations...

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wp · 2012-10-13 10:43

It's even simpler than that because the coordinates of the "back wall"
as passed to the event handler, the extent calculatation therefore is
not needed.

Code: [Select]uses
  Math;
// ...
  p1 := Chart1.XGraphToImage(Max(Chart1ConstantLine1.Position,
ARect.Left));
  p2 := Chart1.XGraphToImage(Max(Chart1ConstantLine1.Position,
ARect.Right));

Maybe you'll have to add 1 to ARect.Left if the left edge of the frame
is painted over.

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madepabloh · 2012-10-13 10:59

Hi WP,

I was writing to you when you included this new option.

So, i observed that the previous solution worked better. Efectively, I
made +1 to the left side and -1 to the right in order to remain the
axis visible. However there was other small problem. It works perfect
except in the case both sides of the rectangle will be outside the
plot area. In that cases, the rentangle is draw outside the plot area
and the vertical axis also dissapear. You can see an image of this
problem below.

However, with your new possible solution, it does not work at all, and
produce the same results: the rentangle goes outside the plot area
when one of the limits is outside...

So, looks like the first solution is better, except for the small
issue i explain above.

EDITED

In the figure, i panned the plot area to the right, so the rectagle
should dessapear by the left side, but is was still drawn outside the
left edge of the plot. The same occurs in the right side. Meanwhile
one of the limits is inside the plot area, the first solution works
perfect.

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wp · 2012-10-13 13:40

QuoteHowever, with your new possible solution, it does not work at
allThe curse of "copy & paste": for p2 the "Max" should be changed to
"Min"...

Quoteworks perfect except in the case both sides of the rectangle will
be outside the plot areaThe curse of "quick & dirty": my code did not
handle all cases correctly. Exit the procedure, when the right bar is
at a position left of the left edge, and for the left bar accordingly.

Maybe this should be better:

Code: [Select]uses
  LCLIntf;  // for rgb function

procedure TForm1.Chart1AfterDrawBackWall(ASender: TChart; ACanvas:
TCanvas;
  const ARect: TRect);
var
  R: TRect;
  p1, p2: integer;
begin
  p1 := Chart1.XGraphToImage(Chart1ConstantLine1.Position);  // left
bar
  if p1 >= ARect.Right then exit;
  p2 := Chart1.XGraphToImage(Chart1ConstantLine2.Position);  // right
bar
  if p2 <= ARect.Left then exit;

  R := Rect(Max(p1, ARect.Left+1), ARect.Top+1, Min(p2,
ARect.Right-1), ARect.Bottom-1);
  ACanvas.Brush.Style := bsSolid;
  ACanvas.Brush.Color := rgb(220, 230, 210);
  ACanvas.FillRect(R);
end;

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Ask · 2012-10-13 15:08

Note that you can also solve the problem in the manner originally
suggested by madepabloh:
Add area series with two points having very large Y value, limit chart
extent using Extent property.

This solution also suffers from the problem of overriding chart frame.
This problem (which also exhibited itself when zooming and panning
various series) should be radically fixed by r39067.

Since r39070, you can also use Infinity instead of arbitrary large Y
value in area series, obviating the need to set chart extent manually.

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madepabloh · 2012-10-13 16:30

@wp,
Yes, i made different changes trying to solve with my very low
knowledge. I captured this issue. In the same way, in the last code it
is also necessary Math declaration in uses.

Thanks so much @wp. Now it works perfect. I tested in different ways,
and works correctly, independently of zooming, panning, resizing of
the chart, change of the constant line series,.... So definitively
[Solved] what i required. It was not the idea i had on mind, but i am
a dummie, so the solution is perfect for me.

@Ask,
Thanks to think about too. Yes, other was the solution i primary
thought about, but just think it was possible to do, because i don´t
know in detail all the possibilities and corners of chart. Thanks for
improve the tool!

So, definitively solved my wish! Thanks folks!