The NSE functions only on weekdays. If we exclude weekends, the number of working days comes down to 261 (365 - 52*2). And then, there comes some more public holidays. This generally varies on each year. The actual working days of the market comes around 250 (+/- 5) days
Y-axis is the days of the year. Each box is filled with Red or Green denoting Negative or Positive movement on that day.