Linechart library for Wio Terminal on PlatformIO IDE needs build_flags = -std=gnu++14

When trying to get the Seeed Arduino Linechart library running with PlatformIO I had some problems as the library didn’t compile.
The reason was that per default gnu++11 compiler was used on my system but gnu++14 was needed.
To get it working the following build_flags had to be set in platformio.ini:

build_unflags = -std=gnu++11
build_flags = -std=gnu++14

Perhaps this should be mentioned in the WiKi and in the ReadMe of the library