This will do the job
clear;
opts = detectImportOptions('2018-12-07--15-55-46 - alt_1.csv','Delimiter',';');
opts = setvartype(opts,[1],'duration'); %as duration
opts = setvartype(opts,[2],'double');
opts = setvaropts(opts,[2],'DecimalSeparator',','); %your file contains `,` as decimal seperator; matlab uses `.` for decimals
T = readtable('2018-12-07--15-55-46 - alt_1.csv',opts); % read in your file
T = [array2table(seconds(T{:,1})) T(:,2)]; %change the HH:mm:ss time format using `seconds`
plot(T{:,1},T{:,2});
If you want to display the values with all decimals in the command window use
format longG
T{1,2}
%>> 0.00145743