Fix examples 1.1 and 1.6

[AlexPishchikov]

This PR fixes filling in examples 1.1 and 1.6.

In example 1.1 filling doesn't match with readme image.
In example 1.6 filling doesn't match with plot legend.

[SiegeLord]

This sounds more like Between option is broken. I'll take a look.

[AlexPishchikov]

Maybe. You use closed gnuplot command for FillRegionType::Between, which fills polygon near line.
For filling between two lines you can use between. It works for me with gnuplot 6.0 but doesn't work properly with gnuplot 5.4.

So, i suggest to add new FillRegionType enum member which will use between command.

[SiegeLord]

Between should already have this behavior. The bug here is that the "closed" evidently at some point produced images like in the readme, but no longer does so. It appears that removing "closed" produces the correct behavior on 5.4 and 6.0. I don't know what "between" does, it is not documented.

So, what's needed here is to adjust the code to not send "closed" to gnuplot in any condition.

[SiegeLord]

This is now fixed by 5025f48.

Thanks for reporting this!