Legend positioning
lazarus / 55-TAChart

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mtanner · 2015-11-01 11:43

The legend Alignment parameter gives basic control over positioning.
Also there is Sidebar, and MarginX/MarginY to refine positioning.

laTopLeft, on a simple conventional X-Y chart result in the legend
obliterating part of the Y axis and scale.  Similarly laBottomCenter
will obliterate the X-axis and scale.

Sidbar is too wasteful of screen space. What I really want to do is to
have Alignment operate relative to the plot area (excluding space
taken up with scales and actual axes). I know I can do this with
MarginX or MarginY, but it's trial and error to know what MarhinX/Y
values are needed to push the legend off the scales.

Is there some parameter I have missed for doing the above?
Alternatively is there a TAGraph function which will calculate the
position of the X-axis (Y-axis) so that this value can be used to set
MarginX/Y?  (There is such a function in TeeChart but I can't find it
in TAGraph).

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wp · 2015-11-01 15:47

Hmm... Legend overlapping with axes if UseSideBar is false seems to be
a bug. I uploaded a fix (r50201). But be aware: UseSidebar=false is
not good as a general setting because the legend still may overlap
with data points.

The chart box surrounded by the axes can be accessed by means of the
property ClipRect of the chart. Note, however, that all the
positioning calculations are done during painting, i.e. ClipRect
changes during the various painting stages. Only in the
OnExtentChanging/OnAfterDraw/OnAfterPaint events the ClipRect does not
change any more - but then, in the latter two, the legend is already
drawn. And I am not sure, but I'd guess that repositioning the legend
in one of these events would trigger another painting run which will
fire these events again, i.e. you will end up with a stack overflow...

So, better stick to the fix mentioned above. I would appreciate if you
could test it because in my own charts I do not work with
UseSidebar=false.

If you do not use the trunk version of Lazarus you should open
TAGraph.pas (it is in (lazarus)/components/tachart), scroll down to
TChart.Draw. In the "try" block, after "PrepareAxis", add the two
lines marked by "new"; then recompile the TAChart package:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TChart.Draw(ADrawer: IChartDrawer; const ARect: TRect);...  try   
PrepareAxis(ADrawer);    if Legend.Visible and not Legend.UseSidebar
then   // <--- new      Legend.Prepare(ldd, FClipRect);              
  // <--- new    if (FPrevLogicalExtent <> FLogicalExtent) and
Assigned(OnExtentChanging) then      OnExtentChanging(Self);   
ADrawer.DrawingBegin(ARect);         ...

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mtanner · 2015-11-01 18:47

Thanks, I will have ago with that.

I generally prefer UseSidBar=false, because UseSideBar=true uses up
space that could oterwise be used to display the chart bigger.  There
is often blank space in the plotting area, so that if the user can
easily control the legend position to the extent of Left/Center/Right
and Top/Middle/bottom then the legend can usually be fitted in withoyt
obscuring any data.

I'm biased by using Latex/PgfPlots, where you can, if you wish,
specify legend position in minute detail, but for routine use PgfPlots
provides:-

 [north west, north, north east, outer north east,
west, east,
south west, south, south east]

All except "outer north east" puts the legend within the plotting
area.  Outer north east being provided for those cases where the legen
in the plot area would obscure that data.