Bug rendering "Progress" shading under some conditions.

To reproduce:

```
from googlegantt import GanttChart, GanttCategory
from datetime import date

gc = GanttChart('Test Chart', width=650, height=200,
progress=date(2011, 02, 27))

on_time = GanttCategory('On Time', '0c0')
late = GanttCategory('Late', 'c00')
upcoming = GanttCategory('Upcoming', '00c')

t1 = gc.add_task('Task1', date(2012, 6, 20), date(2012, 7, 15),
category=on_time)
t2 = gc.add_task('Task2', depends_on=t1, duration=3, category=upcoming)
t3 = gc.add_task('Task3', date(2012,5,01), duration=50, category=late)

url = gc.get_url()
print url
```