Your categories should look like this.
| Quercus rubra | 5-7 | 7-9 | 9-11 | 7-9 | 9-11 | 9-11 | 11-13 | 11-13 | 9-11 | 11-13 | 13-15 | 15-17 |
| Quercus alba | 11-13 | 9-11 | 5-7 | 9-11 | 11-13 | 7-9 | 7-9 | 7-9 | 5-7 | 7-9 | 7-9 | 5-7 |
Now use your graph to plot frequency versus height for just the first two data points of each species. Use open and filled columns to distinguish data from the two species.