BUG: Off by one error in runtime construction of digest panel

When a history minute digest panel was constructed at runtime, there
would be 1 minute missing between the current minute at iloc[-1] and the
next prior minute at iloc[-2].
BUG: Ensure that the bar count is an integer in minutely mode simple
transforms

total_seconds can return a float, which when divided by `60` would
still yield a float causing a value error when passed to range.