Bug: negative bus journeys

[luke-strange]

Buses shouldn’t be able to go “backwards” - i.e. setting the end bus stop as one that is before the start stop on the route.

For example, a bus goes along the route stop1, stop2, stop3... etc.

If i set stop3 as the "start" and stop 1 as the end (by mistake/not understanding the tool correctly), the graph shows negative values on the chart.

See example here

We should not plot the values if they are negative, and perhaps issue a warning to users that they have put their stops the wrong way around.