Why are some of the bars missing on the bpm graph?
Cardiogram reads data from HealthKit, so when things don't show up and there's a gap in the data, it's because either no data was collected by the Apple Watch sensor, or Apple's algorithm determined that the beats-per-minute reading may be inaccurate. This can happen during workouts, since the watch moves quite a bit and can lose contact with skin (e.g. sweat). To get around this, make sure the watch is in contact with your skin (e.g. tighten the band before a workout).
Cardiogram does not limit the number of data points collected per day. In continuous recording mode, a data point is collected every 5 seconds, and in regular mode it is collected every 5 minutes.