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How to create a BSpline or polyfit graph where the x values are datetime objects?

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1

So for a school project I am designing a program that creates a graph of tide heights over time. BSpline doesn't take in datetime objects, and neither does np.linspace which from what I have seen is necessary for BSpline to run.

I have tried using pandas to convert the datetime to a value np can read, or using datetime64. In the code below I have removed all my failed methods and just left the bare bones.

import matplotlib.pyplot as pyplot
from matplotlib.dates import date2num, datestr2num
import numpy as np 
import matplotlib.dates as mdates
from scipy.interpolate import make_interp_spline,BSpline

test_xvals = [datetime(1900, 1, 1, 1, 28), datetime(1900, 1, 1, 5, 47), datetime(1900, 1, 1, 13, 7), datetime(1900, 1, 1, 20, 43)] #times which correspond to the height of the tides
test_yvals = [3.3, 3.9, 0.8, 4.5] #height of the tides in meters

times = date2num(test_xvals) #converts to object compatible with numpy and matplotlib
heights = np.array(test_yvals) #array of 4 floats

x_smooth = np.linspace(times.min(), times.max(), 300)
spl = make_interp_spline(times,heights , k=3) #BSpline object
y_smooth = spl(x_smooth)


fig, ax = plt.subplots()
ax.plot(x_smooth, y_smooth)
ax.scatter(times,heights)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%H:%M'))
#pyplot.xticks(dates,xlabels)
pyplot.ylim(0,6)

The sine graph does not match the data points. What attributes of BSpline should I tweak to make the data points match up with local max/mins?
Graph with out of sync data points

python numpy datetime matplotlib bspline

share|improve this question

edited Mar 27 at 0:32

Inbar AS

asked Mar 26 at 20:33

Inbar ASInbar AS

622 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you expect us to guess where the error occurs?
  
  – hpaulj
  Mar 26 at 20:58
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sorry! Updated it.
  
  – Inbar AS
  Mar 26 at 21:05
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Reread the BSpline docs.
  
  – hpaulj
  Mar 26 at 21:17
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Rereading the BSpline docs was not helpful, but I finally found a stack overflow question that answered a similar issue and got my graph to curve. However, those curves do not line up with the actual data points (I updated the code and included a picture of the output). How can I fix that?
  
  – Inbar AS
  Mar 27 at 0:34
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The thing that stood out to me was that third argument, k, is supposed to be a single number. You gave it an array, from linspace.
  
  – hpaulj
  Mar 27 at 0:49
  

  
  
  
  

 | 
show 1 more comment

1

So for a school project I am designing a program that creates a graph of tide heights over time. BSpline doesn't take in datetime objects, and neither does np.linspace which from what I have seen is necessary for BSpline to run.

I have tried using pandas to convert the datetime to a value np can read, or using datetime64. In the code below I have removed all my failed methods and just left the bare bones.

import matplotlib.pyplot as pyplot
from matplotlib.dates import date2num, datestr2num
import numpy as np 
import matplotlib.dates as mdates
from scipy.interpolate import make_interp_spline,BSpline

test_xvals = [datetime(1900, 1, 1, 1, 28), datetime(1900, 1, 1, 5, 47), datetime(1900, 1, 1, 13, 7), datetime(1900, 1, 1, 20, 43)] #times which correspond to the height of the tides
test_yvals = [3.3, 3.9, 0.8, 4.5] #height of the tides in meters

times = date2num(test_xvals) #converts to object compatible with numpy and matplotlib
heights = np.array(test_yvals) #array of 4 floats

x_smooth = np.linspace(times.min(), times.max(), 300)
spl = make_interp_spline(times,heights , k=3) #BSpline object
y_smooth = spl(x_smooth)


fig, ax = plt.subplots()
ax.plot(x_smooth, y_smooth)
ax.scatter(times,heights)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%H:%M'))
#pyplot.xticks(dates,xlabels)
pyplot.ylim(0,6)

The sine graph does not match the data points. What attributes of BSpline should I tweak to make the data points match up with local max/mins?
Graph with out of sync data points

python numpy datetime matplotlib bspline

share|improve this question

edited Mar 27 at 0:32

Inbar AS

asked Mar 26 at 20:33

Inbar ASInbar AS

622 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you expect us to guess where the error occurs?
  
  – hpaulj
  Mar 26 at 20:58
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sorry! Updated it.
  
  – Inbar AS
  Mar 26 at 21:05
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Reread the BSpline docs.
  
  – hpaulj
  Mar 26 at 21:17
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Rereading the BSpline docs was not helpful, but I finally found a stack overflow question that answered a similar issue and got my graph to curve. However, those curves do not line up with the actual data points (I updated the code and included a picture of the output). How can I fix that?
  
  – Inbar AS
  Mar 27 at 0:34
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The thing that stood out to me was that third argument, k, is supposed to be a single number. You gave it an array, from linspace.
  
  – hpaulj
  Mar 27 at 0:49
  

  
  
  
  

 | 
show 1 more comment

So for a school project I am designing a program that creates a graph of tide heights over time. BSpline doesn't take in datetime objects, and neither does np.linspace which from what I have seen is necessary for BSpline to run.

I have tried using pandas to convert the datetime to a value np can read, or using datetime64. In the code below I have removed all my failed methods and just left the bare bones.

import matplotlib.pyplot as pyplot
from matplotlib.dates import date2num, datestr2num
import numpy as np 
import matplotlib.dates as mdates
from scipy.interpolate import make_interp_spline,BSpline

test_xvals = [datetime(1900, 1, 1, 1, 28), datetime(1900, 1, 1, 5, 47), datetime(1900, 1, 1, 13, 7), datetime(1900, 1, 1, 20, 43)] #times which correspond to the height of the tides
test_yvals = [3.3, 3.9, 0.8, 4.5] #height of the tides in meters

times = date2num(test_xvals) #converts to object compatible with numpy and matplotlib
heights = np.array(test_yvals) #array of 4 floats

x_smooth = np.linspace(times.min(), times.max(), 300)
spl = make_interp_spline(times,heights , k=3) #BSpline object
y_smooth = spl(x_smooth)


fig, ax = plt.subplots()
ax.plot(x_smooth, y_smooth)
ax.scatter(times,heights)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%H:%M'))
#pyplot.xticks(dates,xlabels)
pyplot.ylim(0,6)

The sine graph does not match the data points. What attributes of BSpline should I tweak to make the data points match up with local max/mins?
Graph with out of sync data points

python numpy datetime matplotlib bspline

share|improve this question

edited Mar 27 at 0:32

Inbar AS

asked Mar 26 at 20:33

Inbar ASInbar AS

622 bronze badges

import matplotlib.pyplot as pyplot
from matplotlib.dates import date2num, datestr2num
import numpy as np 
import matplotlib.dates as mdates
from scipy.interpolate import make_interp_spline,BSpline

test_xvals = [datetime(1900, 1, 1, 1, 28), datetime(1900, 1, 1, 5, 47), datetime(1900, 1, 1, 13, 7), datetime(1900, 1, 1, 20, 43)] #times which correspond to the height of the tides
test_yvals = [3.3, 3.9, 0.8, 4.5] #height of the tides in meters

times = date2num(test_xvals) #converts to object compatible with numpy and matplotlib
heights = np.array(test_yvals) #array of 4 floats

x_smooth = np.linspace(times.min(), times.max(), 300)
spl = make_interp_spline(times,heights , k=3) #BSpline object
y_smooth = spl(x_smooth)


fig, ax = plt.subplots()
ax.plot(x_smooth, y_smooth)
ax.scatter(times,heights)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%H:%M'))
#pyplot.xticks(dates,xlabels)
pyplot.ylim(0,6)

share|improve this question

edited Mar 27 at 0:32

Inbar AS

asked Mar 26 at 20:33

Inbar ASInbar AS

622 bronze badges

share|improve this question

edited Mar 27 at 0:32

Inbar AS

asked Mar 26 at 20:33

Inbar ASInbar AS

622 bronze badges

asked Mar 26 at 20:33

Inbar ASInbar AS

622 bronze badges

asked Mar 26 at 20:33

Inbar ASInbar AS

622 bronze badges