If you set the number format on the chart to Auto it can sometimes do better than if you set a specific number format.
Whether you use floor or not shouldn't impact the scale on the chart, but it's usually best not to use floor, as you lose definition if someone exports the data.
If the Auto format isn't working by itself you can use the num function to format the values, and this will affect the axis labels also. You will need to find the highest value in any month to do this though, by using an aggr function, which is a bit fiddly. Letting the chart sort it out itself is generally the best approach.