There are many graphical functions offering the availability of the parameter
alpha which is usually used to specify semi-transparent colors, however, such kind of colors can only be displayed in certain devices, as stated in the help of
Semi-transparent colors (
0 < alpha < 1) are supported only on some devices: at the time of writing only on the
quartzdevices as well as several third-party devices such as those in packages Cairo, cairoDevice, JavaGD and RSvgDevice.
Here is an example illustrating semi-transparent colors in a
You may try other devices such as
bmp, etc, and you will find
alpha is not supported there.