TChart.Extent
lazarus / 55-TAChart

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wp · 2011-07-13 10:44

What is the reason that Extent is a property of TChart? Shouldn't it
be a property of each axis instead (at least the corresponding
1-dimensional part of TChartExtent)? I'm thinking of a situation when,
for example, voltage and current are plotted in the same diagram as a
function of time. Then one would define two vertical axes, the left
one for voltage, the right one for current. But TChartExtent knows
only about one vertical axis. How then would one be able to restrict
the extent of the voltage axis from, say, 0.5 to 1 V, and the extent
of the current axis from 0.1 to 0.2 mA?

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Ask · 2011-07-13 20:51

QuoteWhat is the reason that Extent is a property of TChart?It's
partially historical (it was this way when I started to maintain
TAChart).

However there is also a technical reason:
As you may know, there are three coordinate systems in TAChart -- axis
(aka object), graph (aka world)
and image (aka screen).
TChart.Extent is measured in graph coordinates, while series extents
are measured in axis coordinates.

QuoteBut TChartExtent knows only about one vertical axis. How then
would one be able to restrict the extent of the voltage axis from,
say, 0.5 to 1 V, and the extent of the current axis from 0.1 to 0.2
mA?Strictly speaking, it knows about none of them. But you can use
various axis transformations
(in particular, TAutoScaleAxisTransform). See axistransf demo for some
examples.