Use / or \ in textplot if the function is increasing or decreasing

Before:

```
0.99845 |                                      ... ....
       |                                    ..        ..
       |                                   .            .
       |                                  .              .
       |                                 .                .
       |                                .                  .
       |                               .                    .
       | .                            .                      . 5.55112 |
-.--------------------------.------------------------.-
       |   .                        .                          .
       |    .                      .
       |     .                    .
       |      .                  .
       |       .                .
       |        .              .
       |         .            .
       |          ..        ..
-0.9984 |            ........
         -10                    0                          10
```

After:

```
0.99845 |                                      ... ....
       |                                    ..        ..
       |                                   /            \
       |                                  /              \
       |                                 /                \
       |                                /                  \
       |                               /                    \
       | \                            /                      \ 5.55112 |
-\--------------------------/------------------------\-
       |   \                        /                          \
       |    \                      /
       |     \                    /
       |      \                  /
       |       \                /
       |        \              /
       |         \            /
       |          ..        ..
-0.9984 |            ........
         -10                    0                          10

```
I'm actually undecided as to whether this is actually a good idea or not, so
comments are welcome.