Fix single column graph display

When receiving only a single column of data, no graph would ever show
up.

This was due to the graph display data structure only being
initialized when the number of datasets changes.  However, the dataset
attribute is populated with one entry by default, so when receiving a
single column of data, the graph initialization never happens.

Fix this by initializing the dataset structure to be empty by default.

This bug was introduced in PR #119.

Fixes: #119

Author: Rahix

src/data.rs

@@ -57,7 +57,7 @@ impl Default for DataContainer {
         DataContainer {
             time: vec![],
             absolute_time: vec![],
-            dataset: vec![vec![]],
+            dataset: vec![],
             raw_traffic: vec![],
             loaded_from_file: false,
         }