VCL only:

Internal canvas classes (GDI and GDI+) have been modified extensively. The default graphic canvas for new charts added at design-time is now GDI+ (TGDIPlusCanvas class) instead of GDI (TTeeCanvas3D class).
Switching from one canvas to another can be done at design-time and run-time through the TChart editor dialog (3D -> Render tab), or manually by code:

uses TeeGDIPlus; Chart1.Canvas := TGDIPlusCanvas.Create
uses TeCanvas; Chart1.Canvas := TTeeCanvas3D.Create

An alternative is to use the TeeGDIPlus component:

TeeGDIPlus1.TeePanel := Chart1;
TeeGDIPlus1.Active := True;

Using the TeeGDIPlus component allows persisting settings, like antialiasing and default text quality.

GDI+ offers several features that cannot be done with GDI (or are done better):

Color alpha (transparency)

Any color (pen, font, gradient, brush, etc) is now treated as a proper 32bit color with RGB and Alpha components.
The only limitation is pseudo-colors can't be used (clBtnFace, clWindow, etc).

Setting a color alpha transparency can be done at design-time using the chart editor dialogs.

Using IDE Object Inspector is also possible, but in VCL there is a limitation that TColor is a signed integer, and so values greater than MaxInt (2^31-1) cannot be manually typed (the Object Inspector raises an exception).

Setting colors by code:

Chart1.Legend.Pen.Color := RGBA( 64,64,64, 128) ; // 128 = semi-transparent


Line Antialias

GDI+ has line antialias, which produces much better looking charts, without jagged diagonal and curved lines and polygon boundaries.
There's a speed penalty when using antialias that is not a problem unless you're plotting many many points and/or performing animations.

If not using a TeeGDIPlus component, Antialiasing can be changed at runtime by code like:

TGDIPlusCanvas(Chart1.Canvas).AntiAlias := False


Filled Pens

The base TTeePen class has a new Fill property (of type TTeeBrush) that is only functional when using the GDI+ canvas. Wide borders and lines (of width 2 or more) can now be filled with textures and gradients.

Series1.Pen.Fill.Gradient.Visible := True


Multi-color gradients

The GDI+ canvas allows gradients with more than 3 colors.
The base TTeeGradient class has a new Colors collection property. Each color item in the collection has Offset and Transparency properties:

Chart1.Gradient.Colors.Add( 0.3, clGreen, 50) // 0.3 means 30%, 50% is transparency


Smooth texturing

GDI+ uses brush images to fill polygons, rectangles and rounded rectangles with better speed and output results than GDI.

Transforms

Rotating, translating and scaling elements (like chart titles, legend, series, etc) are enabled using GDI+ transform methods.
Better clipping
When filling polygons with gradients or textures, or when drawing rounded-corner rectangles, clipping should be done manually in GDI (which is not 100% perfect), while in GDI+ this is done automatically and better.
Shadows
In GDI canvas, "smooth" shadows are painted using an intermediary bitmap with a "blur" filter applied. This is a very slow operation, although the output is quite good.
In GDI+, there's another approach to draw smooth shadows that is way much faster with a little loss in quality. Shadows are done using gradients and transparency.
GDI+ speed penalty
There's one thing that is much faster to paint in GDI than GDI+: non-solid pen lines.
Due to this, a big decision has been taken: axis grid lines are now solid instead of dotted:

Chart1.Axes.Left.Grid.Style := psSolid;

Grid lines can easily be changed again to be non-solid, using the axis editor dialog or Object Inspector.

With all the new changes, the GDI+ canvas is now faster than GDI in many occasions, as soon you use any effect like transparency, gradient fills, or shadows.
GDI is still faster than GDI+ when you dont need antialiasing and any fancy effect, for example when plotting FastLine or Surface series with many many points, but the difference between them is now much smaller, and you can always set back to GDI canvas.

FireMonkey Only:

As FireMonkey is already using GDI+ or Direct2D in Windows (and Quartz/OpenGL and the new GPU canvas for Mac OSX and iOS), the above list of GDI+ advantages for VCL are also valid for FireMonkey.
New editor dialog for j@vascript exporting format.

VCL and Firemonkey:

Gradients

TTeeGradient Colors collection property

Gradients can be now configured to use multiple colors in FireMonkey and also in VCL with GDI+ canvas (the limit was three colors maximum with VCL GDI).

Each color item has Color, Offset and Transparency properties.
The Colors property is also used when exporting charts to j@vascript HTML5 and other formats.

New Features:
--------------------------
Full support for RAD Studio, Delphi and C++ Builder 10.3 Rio
for the following platforms is not yet provided due to some
inconsistencies with Rio's own support. This has been
brought to Embarcadero's attention. We will provide an update
as soon as the issues are resolved.

- Delphi FMX for iOS Simulator.
- C++Builder FMX for MacOSX and Android.
- Delphi FMX for iOS Device 32bit and iOS Device 64bit
- C++Builder FMX for iOS Device 32bit and iOS Device 64bit