	Generalities
	Curve data Organizing the display area Pixel and data coordinates To handle Curve plotting, two classes were developed : `jephem.tools.Curve`, handling the core curve functionnalities and the display (it is a subclass of `JPanel`). `jephem.gui.CurveViewer`, which handles the commands

Curve data

Curve API provides 2D representation. So it has an X axis and an Y axis.
To display a curve, client classes send double arrays. The display is done from discrete values. Typically, 100 values are enough to get a continuous impression on the screen.

One array is needed to represent X values.

As Curve API can display several curves on the same drawing, it needs one double array per represented curve.

X and Y values are stored in a double[][], accessible via getData().
This data are given to Curve via the constructor (see Curve javadoc).

public int getNbX() permits to retrieve the number of values used to represent each curve.

public int getNbY() permits to retrieve the number of represented curves.

getXMin(), getXMax(), getYMin(), getYMax() are also available.

Other way to access the data :

public double[] getX(), to get the X values.

public double[] getY(int whichY), to get the values of the whichYth curve.

public double[][] getAllY(int whichY), to get the values of all the curves.

getY(int whichY, double x) permits to retrieve the ordinate corresponding to x for th whichYth curve. An interpolation is done if x does not correspond to one of the pre-calculated values (see interpolation).

Organizing the display area

Here, we only consider the area handled by Curve. It is organized like this :

The gaps are used to put legends. There are top, left, bottom, right gaps, called tg, lg, bg, rg.
Other useful quantities are h and w, the height and width of the area handled by Curve.

Pixel and data coordinates

Here, it is useful to consider two types of coordinates :

Data coordinates, noted (x_D, y_D), expressed in the data scale.
We have : XXXXX

Pixel coordinates, noted (x_P, y_P), expressed in the usual computer screen coordinates (origin at top - left);
We have : XXXXXX

We also note : Dx = x_Max - x_Min and Dy = y_Max - y_Min

Data to pixel

Data and pixel coordinates are linked by a linear relation.

For x, we have : x_P = a₁x_D + b₁ ( relation 1).

To find a₁ and b₁, we need to see the values for two different points :

x_D	x_P
x_Min	lg
x_Max	w - rg

This leads to a system of two equations with two unknown ; the solutions are :
XXXXXXXXX

For y, we have : y_P = a₂y_D + b₂ (relation 2).

The same method is applied ; we have :

y_D	y_P
y_Min	h - bg
y_Max	ug

The solutions are :
XXXXXXXXXXXXXXXXXX

Pixel to data

If we pose : x_D = a'₁x_P + b'₁, from relation 1 we deduce :
a'₁ = 1 / a₁ and b'₁ = - b₁/a₁
So : XXXXXXXX

For y, we pose : y_D = a'₂y_P + b'₂ ; the same method leads to
XXXXXXXXXX

This is implemented in private methods pixelToData() and dataToPixel().

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	JEphem Curve plotting Generalities