Angular $scope and vars from select box confusion - angularjs

Question

I have setup a system where I can filter displaying some reporting data on button clicks and drop down list selections.
The filtering works well on the button clicks but I am running into trouble with the drop down list.
Setup is as follows:
... more buttons above with the same as below
<div class="col col-center">
<button class="button button-outline button-positive" ng-click="setMetric('2')">Personal</button>
</div>
<div class="col col-center">
<button class="button button-outline button-positive" ng-click="setMetric('3')">Business</button>
</div>
</div>
<div class="list">
<label class="item item-input item-select">
<div class="input-label">
Area
</div>
<select ng-options="z.id as z.zone for z in zones" ng-model="area"></select>
</label>
</div>
<p>Ref: 1.{{category}}.{{metric}}.0.{{area}}</p> <!-- this updates fine -->
<div ng-show="shouldShow(1.1.1.0.1)" line-chart chart-data="data"></div>
<div ng-show="shouldShow(1.1.1.0.2)" line-chart chart-data="data2"></div>
and the controller:
$scope.category = '1';
$scope.metric = '1';
$scope.area = '1';
$scope.zones = [
{'zone':'National', 'id':'1'},
{'zone':'QLD', 'id':'2'},
{'zone':'All other areas', 'id':'3'},
{'zone':'Affluent', 'id':'4'},
{'zone':'Mass', 'id':'5'}
];
$scope.setCategory = function(category) {
$scope.category = category;
};
$scope.setMetric = function(metric) {
$scope.metric = metric;
};
$scope.setArea = function(area) {
$scope.area = area;
};
$scope.getArea = function() {
return $scope.area;
};
$scope.shouldShow = function () {
console.log("1." + $scope.category + "." + $scope.metric + ".0." + $scope.area); // This does not
switch ("1." + $scope.category + "." + $scope.metric + ".0." + $scope.area) {
case "1.1.1.0.1":
return true;
break;
case "1.1.1.0.2":
return true;
break;
default: return false;
}
};
The problem I am running into is that the area var on the page (see the html comment as part of Ref: 1.{{category}}.{{metric}}.0.{{area}}) does update but the reference to it in the function shouldShow() does not update.
How am i misunderstanding scope and how do I make this work?