update implementation-nodes.md, see #43

doc/implementation-notes.md

@@ -44,7 +44,7 @@ PhET-specific terminology:
 
 This section describes how this simulation uses patterns that are common to PhET simulations.
 
-**Model-view transform**: This simulation has a model-view transform that maps from the graph (model) coordinate frame to the view coordinate frame. The origin (0,0) in the model cordinate frame is (not surprisingly) at the orgin of the graph.
+**Model-view transform**: This simulation has a model-view transform that maps from the graph (model) coordinate frame to the view coordinate frame. The origin (0,0) in the model coordinate frame is (not surprisingly) at the origin of the graph.
 
 **Query parameters**: Query parameters are used to enable sim-specific features, mainly for debugging and
 testing. Sim-specific query parameters are documented in
@@ -82,6 +82,6 @@ This section provides an overview of the most important view components.
 
 Each screen has a class whose name ends with "InteractiveEquationNode", for example, [ExploreInteractiveEquationNode](https://github.com/phetsims/graphing-quadratics/blob/master/js/explore/view/ExploreInteractiveEquationNode.js). This is the UI for modifying the interactive quadratic indirectly, using sliders or pickers. _Explore_ and _Focus & Directrix_ screens use sliders, _Standard Form_ and _Vertex Form_ screens use pickers.
 
-Manipulators are the shaded spheres that appear on the graph. They allow the user to modify the interactive quadratic directly, by interacting with its curve. The [Manipulator](https://github.com/phetsims/graphing-lines/blob/master/js/common/view/manipulator/Manipulator.js) base class is reused from the Graphing Lines sim, and is extended in the [GQManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/common/view/GQManipulator.js) base class, which adds optional (x,y) coordinates.  The 3 subclasses of `GQManipulator` are [VertexManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/common/view/VertexManipulator.js), [FocusManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/focusanddirectrix/view/FocusManipulator.js) and [PointOnParabolaManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/focusanddirectrix/view/PointOnParabolaManipulator.js), for changing the vertex, focus, and point on the parabola respectively.
+Manipulators are the shaded spheres that appear on the graph. They allow the user to modify the interactive quadratic directly, by interacting with its curve. The [Manipulator](https://github.com/phetsims/graphing-lines/blob/master/js/common/view/manipulator/Manipulator.js) base class is reused from the Graphing Lines sim, and is extended in the [GQManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/common/view/GQManipulator.js) base class, which adds optional (x,y) coordinates.  The 3 subclasses of `GQManipulator` are [VertexManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/common/view/VertexManipulator.js), [FocusManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/focusanddirectrix/view/FocusManipulator.js) and [PointOnParabolaManipulator](https://github.com/phetsims/graphing-quadratics/blob/master/js/focusanddirectrix/view/PointOnParabolaManipulator.js), for changing the vertex, focus, and point on the parabola, respectively.
 
 Each checkbox in this sim is a subclass of [GQCheckBox](https://github.com/phetsims/graphing-quadratics/blob/master/js/common/view/GQCheckbox.js), a subtype of `SUN/Checkbox` that supports labeling with an optional icon, and provides standardization of various properties (font, fill,...) You can locate a `Checkbox` subclass based on the English text used on checkbox. For example, [FocusCheckbox](https://github.com/phetsims/graphing-quadratics/blob/master/js/focusanddirectrix/view/FocusCheckbox.js) is the checkbox labeled with "Focus" and a focus manipulator icon.