TAChart has two levels of drawing abstraction.
First level is a drawing back-end. It allows to draw charts
on various media and/or file formats.
For example, you can generate charts as SVG files, print them,
draw them in OpenGL context etc.
http://wiki.lazarus.freepascal.org/TAChart_documentation#DrawersSome back-ends, in turn, support several output formats themselves,
for example FPVectorial back-end can save chart in vector formats
such as DWG and CorelDraw.
The default drawing back-end is LCL canvas, which supports various
widgetsets. TAChart (almost) does not care about the specific
widgetset, it just uses TCanvas interface. So whatever is implemented
by TCanvas works in TAChart.
Based on the above you have two options for porting TAChart:
1) Add the missing features to CustomDraw widgetset
(for example, from my brief testing it seems that it does not currently support
line width > 1).
2) Use some other back-end, such as SVG or OpenGL.
3) Failing all the above, write Android-specifick back-end.
I am interested in porting TAChart to Android, but currently
lack time and equipment for testing on that platform.
However, if the first option will be feasible for you,
then my task will be much simpler -- I will just test
TAChart on CustomDrawn widgetset
and maybe help to implement what's missing there.