[Lazarus] TAchart multiple reticules and vertical series alignment

[Alexander Klenin]

On Sat, Jun 5, 2010 at 00:02, Rick <rick at sloservers.com> wrote:
>>> I'm having a little bit of trouble with my axis labels.  TAChart seems to
>>> only want to display even numbers on the horizontal axis, for example.
>>> Even when I use a custom source and set it to X values, it only shows
>>> the even numbers.  What am I missing?
>>>
>>
>> I am not sure, seems to work for me.
>> Can you post a project demonstrating the problem?
>>
>
> Well since we're talking about it, let's take the line demo, for example.
> The horizontal axis displays 0, 5, 10, ... What would I have to do to make
> it display 0,1,2,3,4,5,6,7,8,9,10... ?
> I've tried everything I can think of including giving the axis its own source
> and changing the values, but it seems that something else is deciding what
> numbers to actually print.

With linedemo:
1) Add TListChartSource component
2) Edit its DataPoints property, enter 1,2,...,10 on a separate lines
3) Set Marks.Source for bottom axis to that source
4) Set Marks.Style to smsXValue
Of course, you can use UserDefinedSource instead for better efficiency.

>> How many points per serie do you have?
>>
> Well this is what seems to make the difference.  I have 10,240 points per series.
> I tried it with the line demo, as well.  Throw a vertical line and drag tool on the chart, then
> drag the line around.  Then click the button to add a bunch of series and try dragging
> the line.  It starts to slow even at 1000 points per series, but panning and dragging become
> unacceptable at 10,000 points per series. (I just got your other email and
> tried resizing the window.  The slowdown is barely noticeable
> at 1000pts per series, but quite obvious at 10,000 points)
>
> I did implement a user-defined series that manually retrieves the data and
> draws the lines.
> That works much better, but it's a lot wasted/duplicate effort.
> It seems like data access might be at the heart of the issue, since the
> problem scales with the
> number of data points in the series.  I wonder if a sorted data source that
> does a binary search would be the ticket.

On the one hand, that is ridiculously large dataset -- averaging more than
five data points per x-axis pixel (!).
On the other hand, you are not the first user who does this,
so I guess there is some reason for such datatsets ;-)

I have added simple time measurement to linedemo and
performed some experiments. It turned out that the most expensive call
is ACanvas.Line itself. Fortunately, I was able to transform drawing algorithm
to split the series into a set of poly-lines, and then draw those.
Unfortunately, it cost a lot of code complexity.
The drawing time is reduced by approximately 3x.
You can measure the speedup by comparing with 'Origin' line type, which
does not benefit from that optimization.
_Now_ the data access/transformation is indeed
the most expensive operation remaining.
I did another minor optimization to bypass transformation calls in
trivial case, which gives another 10-15% speedup.

Further optimization is possible by caching the poly-lines.
However, the complexity will raise even further.
So before we go there, please test the current version -- maybe
it is already good enough?

-- 
Alexander S. Klenin