char_speed  start_time  end_time
0         34           3        15
1         19          15        21
2          9          21        28
...

start_time  end_time  speed
0       9.23    20.929    1.0
1        1.4     20.26    1.5
2       20.0      27.6   1.25
...

start_time  end_time  speed  total_char
0       9.23    20.929    1.0
1        1.4     20.26    1.5
2       20.0      27.6   1.25
...

((15-9.23)*34) + ((20.929-15)*19)

(3-1.4)*0 + ((15-3)*34) + ((20.26-15)*19)