data cursor

Development matplotlib-devel
1

Hi folks,

I am planning to re-write a data viewer program I wrote ages ago using C + Xlib.

This program allows me to plot on the same window several graphs (time
versus brightness - I work on variable stars). They all share the
same independent variable (time passes the same for everybody).

I can scroll and zoom the graphs both in X and Y. When I scroll/zoom
in X all plots suffer the same action. Y scrolling/zooming may be
done for each plot.

Now the question: I need a data cursor, by that I mean some marker
that sits on top of a point and which may be moved forward/backward by
pressing a key. Matplotlib has a mouse cursor that gives me the
cursor coordinates, but it doesn't

- sit on top of my points and therefore doesn't give me the exact
value of that point
- allow me to move from one point to the next
- allow me to change which graph I want the cursor sitting on

I thought of drawing my own cursor, deleting it and moving to the next
point . It seems this would take forever as (far as I understand)
matplotlib will redo the entire plot each time I do this.

How hard is it for the developers to include a built in data cursor in
a similar fashion to the mouse cursor now available. I am affraid
this isn't too easy, I don't know any plotting program that has one
like what I need.

thanks for your attention,

Antonio Kanaan

2

Hi folks,

I am planning to re-write a data viewer program I wrote ages ago using C + Xlib.

This program allows me to plot on the same window several graphs (time
versus brightness - I work on variable stars). They all share the
same independent variable (time passes the same for everybody).

I can scroll and zoom the graphs both in X and Y. When I scroll/zoom
in X all plots suffer the same action. Y scrolling/zooming may be
done for each plot.

Now the question: I need a data cursor, by that I mean some marker
that sits on top of a point and which may be moved forward/backward by
pressing a key. Matplotlib has a mouse cursor that gives me the
cursor coordinates, but it doesn't

This isn't too bad actually -- you can subclass the existing Cursor
class to override the xdata and ydata atrributes to insure that the
cursor sits only on your data points. The example below uses the
cursor class to overrride the toolbar formatting, and shows you how to
toggle the visibility of multiple cursors in multiple axes with shared
x axes. I'll also attach it in case the mail system mangles the
newlines. Take a look at the MultiCursor in
the widgets module, you might be able to do a similar trick there to
have common x cursoring across multiple axes.

from pylab import figure, show, nx
from matplotlib.widgets import Cursor

class DataCursor(Cursor):
    def __init__(self, t, y, ax, useblit=True, **lineprops):
        Cursor.__init__(self, ax, useblit=True, **lineprops)
        self.y = y
        self.t = t
        self.xstr = ''
        self.ystr = ''

    def onmove(self, event):
        """
        we override event.xdata to force it to snap-to nearest data
        item here we assume t is sorted and I'll use searchsorted
        since it is a little faster, but you can plug in your nearest
        neighbor routine, eg to grab the closest x,y point to the
        cursor
        """
        xdata = event.xdata
        ind = nx.searchsorted(self.t, xdata)
        ind = min(len(self.t)-1, ind)
        event.xdata = self.t[ind]
        event.ydata = self.y[ind]
        self.xstr = '%1.3f'%event.xdata
        self.ystr = '%1.3f'%event.ydata
        Cursor.onmove(self, event)

    def fmtx(self, x):
        return self.xstr

    def fmty(self, y):
        return self.ystr

fig = figure()
ax1 = fig.add_subplot(211)
ax2 = fig.add_subplot(212, sharex=ax1) # connect x pan/zoom events

t = nx.cumsum(nx.rand(20))
s1 = nx.mlab.rand(len(t))
s2 = nx.mlab.rand(len(t))

ax1.plot(t, s1, 'go')
ax2.plot(t, s2, 'bs')
ax1.set_title("Press 1 for upper cursor and 2 for lower cursor")
cursor1 = DataCursor(t, s1, ax1, useblit=True, color='red', linewidth=2 )

# we'll let the cursor do the toolbarformatting too.
ax1.fmt_xdata = cursor1.fmtx
ax1.fmt_ydata = cursor1.fmty

cursor2 = DataCursor(t, s2, ax2, useblit=True, color='red', linewidth=2 )
ax2.fmt_xdata = cursor2.fmtx
ax2.fmt_ydata = cursor2.fmty

# now we'll control the visibility of the cursor; turn off cursor2 by default
cursor2.visible = False

def keyevent(event):
    cursor1.visible = event.key=='1'
    cursor2.visible = event.key=='2'
    fig.canvas.draw()

fig.canvas.mpl_connect('key_press_event', keyevent)
show()

data_cursor.py (1.91 KB)

···

On 2/26/07, Antonio Kanaan <ankanaan@...149...> wrote:

- sit on top of my points and therefore doesn't give me the exact
value of that point
- allow me to move from one point to the next
- allow me to change which graph I want the cursor sitting on

I thought of drawing my own cursor, deleting it and moving to the next
point . It seems this would take forever as (far as I understand)
matplotlib will redo the entire plot each time I do this.

How hard is it for the developers to include a built in data cursor in
a similar fashion to the mouse cursor now available. I am affraid
this isn't too easy, I don't know any plotting program that has one
like what I need.

thanks for your attention,

Antonio Kanaan

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3

I forgot to mention in my previous post -- as long as you use the blit
techniques embodied in the Cursor class in the example above and
discussed here:

http://www.scipy.org/Cookbook/Matplotlib/Animations

matplotlib will not redraw you figure each time.

JDH

···

On 2/26/07, Antonio Kanaan <ankanaan@...149...> wrote:

I thought of drawing my own cursor, deleting it and moving to the next
point . It seems this would take forever as (far as I understand)
matplotlib will redo the entire plot each time I do this.

4

Hi John,

thanks *very much* for your message. This is exactly what I want.
And it is going to be a lot less work than it was in C / Xlib. Even
re-reading that code was a nightmare, adding new features would be
nearly impossible :slight_smile:

I will now start redoing all my code using matplotlib.

cheers

Antonio

Antonio Kanaan
Departamento de Fisica - Universidade Federal de Santa Catarina
CP 476 -- CEP 88040-900 -- Florianopolis -- SC -- Brasil
http://www.astro.ufsc.br/~kanaan e-mail: kanaan@...510...
Phone: 55,48,33319069 FAX : 55,48,33319946

···

On 2/26/07, John Hunter <jdh2358@...149...> wrote:

On 2/26/07, Antonio Kanaan <ankanaan@...149...> wrote:
> Hi folks,
>
> I am planning to re-write a data viewer program I wrote ages ago using C + Xlib.
>
> This program allows me to plot on the same window several graphs (time
> versus brightness - I work on variable stars). They all share the
> same independent variable (time passes the same for everybody).
>
> I can scroll and zoom the graphs both in X and Y. When I scroll/zoom
> in X all plots suffer the same action. Y scrolling/zooming may be
> done for each plot.
>
> Now the question: I need a data cursor, by that I mean some marker
> that sits on top of a point and which may be moved forward/backward by
> pressing a key. Matplotlib has a mouse cursor that gives me the
> cursor coordinates, but it doesn't

This isn't too bad actually -- you can subclass the existing Cursor
class to override the xdata and ydata atrributes to insure that the
cursor sits only on your data points. The example below uses the
cursor class to overrride the toolbar formatting, and shows you how to
toggle the visibility of multiple cursors in multiple axes with shared
x axes. I'll also attach it in case the mail system mangles the
newlines. Take a look at the MultiCursor in
the widgets module, you might be able to do a similar trick there to
have common x cursoring across multiple axes.

from pylab import figure, show, nx
from matplotlib.widgets import Cursor

class DataCursor(Cursor):
    def __init__(self, t, y, ax, useblit=True, **lineprops):
        Cursor.__init__(self, ax, useblit=True, **lineprops)
        self.y = y
        self.t = t
        self.xstr = ''
        self.ystr = ''

    def onmove(self, event):
        """
        we override event.xdata to force it to snap-to nearest data
        item here we assume t is sorted and I'll use searchsorted
        since it is a little faster, but you can plug in your nearest
        neighbor routine, eg to grab the closest x,y point to the
        cursor
        """
        xdata = event.xdata
        ind = nx.searchsorted(self.t, xdata)
        ind = min(len(self.t)-1, ind)
        event.xdata = self.t[ind]
        event.ydata = self.y[ind]
        self.xstr = '%1.3f'%event.xdata
        self.ystr = '%1.3f'%event.ydata
        Cursor.onmove(self, event)

    def fmtx(self, x):
        return self.xstr

    def fmty(self, y):
        return self.ystr

fig = figure()
ax1 = fig.add_subplot(211)
ax2 = fig.add_subplot(212, sharex=ax1) # connect x pan/zoom events

t = nx.cumsum(nx.rand(20))
s1 = nx.mlab.rand(len(t))
s2 = nx.mlab.rand(len(t))

ax1.plot(t, s1, 'go')
ax2.plot(t, s2, 'bs')
ax1.set_title("Press 1 for upper cursor and 2 for lower cursor")
cursor1 = DataCursor(t, s1, ax1, useblit=True, color='red', linewidth=2 )

# we'll let the cursor do the toolbarformatting too.
ax1.fmt_xdata = cursor1.fmtx
ax1.fmt_ydata = cursor1.fmty

cursor2 = DataCursor(t, s2, ax2, useblit=True, color='red', linewidth=2 )
ax2.fmt_xdata = cursor2.fmtx
ax2.fmt_ydata = cursor2.fmty

# now we'll control the visibility of the cursor; turn off cursor2 by default
cursor2.visible = False

def keyevent(event):
    cursor1.visible = event.key=='1'
    cursor2.visible = event.key=='2'
    fig.canvas.draw()

fig.canvas.mpl_connect('key_press_event', keyevent)
show()

>
> - sit on top of my points and therefore doesn't give me the exact
> value of that point
> - allow me to move from one point to the next
> - allow me to change which graph I want the cursor sitting on
>
> I thought of drawing my own cursor, deleting it and moving to the next
> point . It seems this would take forever as (far as I understand)
> matplotlib will redo the entire plot each time I do this.
>
> How hard is it for the developers to include a built in data cursor in
> a similar fashion to the mouse cursor now available. I am affraid
> this isn't too easy, I don't know any plotting program that has one
> like what I need.
>
> thanks for your attention,
>
> Antonio Kanaan
>
> -------------------------------------------------------------------------
> Take Surveys. Earn Cash. Influence the Future of IT
> Join SourceForge.net's Techsay panel and you'll get the chance to share your
> opinions on IT & business topics through brief surveys-and earn cash
> http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
> _______________________________________________
> Matplotlib-devel mailing list
> Matplotlib-devel@lists.sourceforge.net
> matplotlib-devel List Signup and Options
>