improve datetime support

Currently the ability to use datetime in kandy-lets-plot (using it as a datatype as well as customisation of scales, guides, etc.) is limited due to the lack of full support in Lets-Plot engine. It is necessary to close all the problems in it.

Start by surveying kandy-lets-plot's current datetime handling and the corresponding Lets-Plot engine limitations. Map the affected datatype, scales, guides, and other customization areas before defining the scope. Done means the identified datetime support problems are closed and the affected behavior is verified.

Written by the indexing model from the issue text.