Data sets from TSV files with NAN aren't displayed correctly

[ghorwin]

Steps to reproduce:

Create TSV-File with the following content

Datum/Zeit	A [-]	B  [-]	C  [-]
2020-01-01 08:00	1	NAN	1
2020-01-01 09:00	2	NAN	2
2020-01-01 10:00	3	2	3
2020-01-01 11:00	4	NAN	4
2020-01-01 12:00	5	2	5
2020-01-01 13:00	NAN	NAN	1
2020-01-01 14:00	NAN	2	2
2020-01-01 15:00	NAN	2	3
2020-01-01 16:00	NAN	2	4
2020-01-01 17:00	NAN	2	5
2020-01-01 18:00	NAN	NAN	1
2020-01-01 19:00	NAN	22	2
2020-01-01 20:00	NAN	2	3
2020-01-01 21:00	NAN	2	4
2020-01-01 22:00	NAN	2	5
2020-01-01 23:00	NAN	NAN	1
2020-01-02 00:00	NAN	NAN	2
2020-01-02 01:00	10	NAN	3

(see attached file).

Data points with NAN are positioned always at time 0.

[ghorwin]

see #289

[ghorwin]

Milestone renamed