fix title and labels when chaining

currently if the user does not specify title, xlabel, or ylabel and the graph is being chained, the resulting output uses the last xcol, ycol values

i.e.

```
graph samples/sine.csv -C | graph samples/cosine.csv
```

gives title `cos(2πx/1000) vs x` and it should be `sin(2πx/1000), cos(2πx/1000) vs x` as is the case with

```
graph samples/sine-cosine.csv
```