Error with different format of "from" and "until"

When one was defined with "-2d" and the other with "HH:mm_YYYYMMDD"
a error occured because the datetime objects produced weren't comparable.
This patch make them comparable whatever their format
Patch for the constantLine function

When we wanted to draw a graph with just a constantLine,
it raised a ZeroDivisionError.
This patch corrects this error.
Add a function "callbackToNode"

Doc and example in the code