I would aim for 10+ seconds for sending data from any MQTT device. I don’t have any reason to say a feature like the line chart wouldn’t work with more frequent samples, just that our Engineering Manager said he would throttle people if they were sending data more than once a second. 
That being said, about the line chart:
-
It is expected that the “Live” tab is missing for MQTT and LoRa connected devices, nothing has gone wrong here, this is simply a design choice and partially reflective of the fact that we’re not expecting these devices to update so frequently. The Live tab should still appear for Pi and Arduino devices connected through their own connectivity options in the Cayenne UI. It may re-appear for MQTT/LoRa devices in the future but I’m not aware of any immediate plans to do so.
-
I expect that the “No Data Available for this period” message should clear as soon as there is sufficient data in each of the tab views. For the hour view this should be no more than 60 minutes, and for the day view no more than 24 hours, and so on. If you’re still seeing a blank line chart and your device has been connected sending data for longer than those times, we might be onto some sort of a bug.
If you find one or more of your MQTT widgets in that state, it would help me to troubleshoot if you could try the steps in this post to extract the data that we have stored for that widget and shared with me at support@mydevices.com so I can investigate. Or PM me with your account credentials and a mention of the problem widget(s) and I’d be happy to extract/investigate myself.