plotyy is not recommended. Instead, use yyaxis which replaces plotyy. I've converted your code below. yyaxis does not have the problem you've found in plotyy.
data1 = [1 2 3 2.5 5 6 7];
data2 = [10 20 30 25 50 60 70];
Timestamp_datetime1 = datetime(2014,1:7,1);
Timestamp_datetime2 = datetime(2014,2:8,1);
figure
yyaxis left
hLine1 = plot(Timestamp_datetime1, data1,'o');
yyaxis right
hLine2 = plot(Timestamp_datetime2, data2,'--');
