plot data in from structure of data points with limits

I have a structure of datapoints, each of which is a structure of variables. I have created a simple function that will do a scatter plot all of a given x and y variable for each datapoint (see below). I would like to be able to provide a limiting variable so that not necessarily all of the data points will be plotted (ie, plot x vs y where z<10). The syntax that I'm pictureing is something like plot_data(array,'x','y','z','<10') but I'm not sure how to implement this.

function plot_data(array,xVar,yVar,varargin)

% Plots data from datapoints or datapoints_onesweep array
% 
% Inputs:
%     array   = datapoint array to plot from
%     xVar    = x-variable for plot
%     yVar    = y-variable for plot

if nargin > 3
    limitVar = varargin{1};
    limitVal = varargin{2};
end

f = fieldnames(array);
d = cell(0);
x = [];
y = [];

for i = 1:length(f)
    if strcmpi(f{i}(1),'D') && strcmpi(f{i}(2),'P')
        d = [d f{i}];
        x = [x array.(f{i}).(xVar)];
        y = [y array.(f{i}).(yVar)];
    end
end

h = scatter(x,y);

I know that I can add:

eval(['array.(f{1}).(limitVar)' limitVal])

to my if statement, but I'd like to avoid using eval if at all possible. Any ideas?