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We just finished setting up our
menu to give users a choice in

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how much data they'd like to see.

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But while the menu is there,
the functionality isn't.

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We still need to create
the showLast function.

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So let's use Alt+Enter to create it.

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And I'm going to change
the variable name to n.

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Now that we've got our showLast function,
we just need to make it work.

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Which means in the onDraw function,
instead of drawing all of our data,

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we need to be able to
draw just a subset of it.

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Let's start by creating a new field
to hold a subset of our data.

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Let's use Cmd or
Ctrl+D to duplicate our data line, and

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then just change the name to subset.

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Then inside onDraw, let's use the subset
of our data instead of all the data.

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So let's replace data with subset,
data with subset, and data with subset.

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Finally, in the constructor,

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let's initialize our subset
to be equal to our data.

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Subset equals data.

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Nice.

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Now that we're using a subset of
our data instead of all the data,

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we just need to update the subset and
the showLast function.

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So inside the showLast function,

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let's just set subset
equal to data.subList(0,

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n), and there we go.

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All that's left is implementing the zero
parameter version for the all data option.

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So let's just copy and
paste our showLast function.

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And then remove the parameter.

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Then inside this function,
let's just call the other

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showLast function while
passing in data.size for n.

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Awesome, now let's run the app and
see what happens.

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We've got our chart, and
we've got our options.

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And if we click on one, nothing happens.

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But don't worry, I know what we did wrong.

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As it turns out,
drawing is kind of an expensive operation.

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An Android does its best to
avoid any unnecessary drawing.

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So whenever you make a change
to one of your views,

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we need to tell Android about it, which
we do by using the invalidate method.

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Calling invalidate on a view signals to
Android that this view is no longer valid

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and needs to be redrawn.

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Also, along with invalidate,

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you should probably know about
the request layout function.

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If you change the size of view, you
want to use the request layout function.

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And if you change both
the content of the view and

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the size, then you'll need to call
both invalidate and request layout.

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So at the bottom of our original showLast
function, let's add a call to invalidate.

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And if we run the app again.

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And then pick one of the options.

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Perfect!

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Instead of all the data,
we've just got the past year.

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And if we click through the rest of these
options, it looks like they all work.

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Though, since we aren't changing
any of the y coordinates for

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something like past week,
it pretty much just looks like a line.

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We'll fix that in the next video.