We just released our free and open source tool for log analysis. Actually, its third version already. It is very useful for embedded applications in general and IoT in particular.
See it here:
https://vcdmaker.orgIt allows for converting your logs to the VCD (Variable Change Dump) format files and visualizing them afterwards. It can parse any text format. All you need to do is to provide the description of your log syntax in the XML format. There's manual available along with some examples.
One can also merge log files coming from different devices (whether they are synchronized or not). The log files might be even taken in different logs

I encourage you to give it a try.