Looks like your date field is discrete. Right click on the pill and change it to continuous.
Couple of things to note:
1- You are deriving "Start Journey YYYYMMDD" as a string. Once you do this it is no more a date field and you can not have missing dates in the graph.
2- You can convert the format of the date to see that in that particular format as below:
And choose custom format
Now you can use this field instead of your derived date field. Also on the pill make this as discrete field. Now you will see missing dates also.
Ok So basically it will show only the missing dates between your minimum and maximum date.
Means if you have data for 2nd Jan and 30th Jan. it will show all the bars from 2nd,3rd,4th till 30th Jan but not for 1st Jan and 31st Jan.
Your advise so helpful for me.