Graph Plotting - Plotting gaps in data as a set value

Hello all !

I have a data set from which I need to produce a simple x-y graph. The graph will be Time (Decimal Form - Ex. 40502.93750000) against an Oxygen level value. I have around 800 data points in an excel document.

My problem lies in that my data points have been taken at random time intervals, what I need to show is that whenever I don't have an oxygen value that a value of something like (-1) is plotted. I realise that this will result in the graph being a bunch of spikes, but this is what I'm looking for.

Any help is greatly appreciated,

Andrew

clc
clear all

    data1 = xlsread('sheet1.xlsx');     % Datasheet 1 import

    x1 = data1(:,4);                    % Setting Spreadsheet columns to be imported
    y1 = data1(:,5);

        plot(x1,y1,'r');                % Plot Spreadsheet columns

[Roger]

may u show some data please ?